Achievement

An Achievement is a reward bestowed and given to players when they accomplish notable gaming feats and obstacles in Windows Games for Vista and the Xbox 360 sets of games.

An Achievement also endows upon the player Gamerpoints that count towards a gamer's total Gamerscore. These points, while not redeemable, are intended to serve as a tangible form of special rewards for unusual or skilled actions within game play. (http://www.xbox.com/en-US/games/tips/achievements.htm)

Various achievements unlock armor types in Halo 3. In Halo 3, there are 79 achievements to unlock and a total of 1,750 Gamerscore to earn.

On September 2, 2008, A user by name Cocopjojo  post pictures from PAX showing new achievements

Comparisons And Contrasts Between Halo 2 Vista's And Halo 3's Achievement Lists
Note: The Xbox version and specific variant of Halo 2, since it was released long before the Xbox 360, contains no achievements.

Both games' respective lists are similar in that in both cases, achievements are split up fairly evenly between single-player and multiplayer modes.

Halo 3, however, offers quantitative rewards for finding the various Skulls located throughout the game, while Halo 2 does not.

"Kill three (or four) enemies within four seconds of each other" comprises a pair of achievements common to both titles. While Halo 2 PC refers to these achievements as "Triple Kill" and "Killtacular" respectively, however, Halo 3 refers to these as "Triple Kill and Overkill", because Overkill is referred to as "Killtacular" in Halo 2.

Halo 3 offers several achievements ("Used Car Salesman" and "Headshot Honcho", for example) that may be earned in either the single-player Campaign or online Multiplayer, while Halo 2 doesn't at all.

Halo 3 Achievements
This is a list of Halo 3 Achievements. There are a total of 49 original achievements that were shipped with the game, worth a total of 1000 Gamerscore (GS), with another 30 DLC(Downloadable Content) achievements worth 750 GS. Some achievements unlock certain armor permutations.

* Bungie later announced in a weekly update that, to receive this achievement, you actually have to reach 7 EXP on the 7th of any month, not enter with 7 EXP.

Complete Online Achievement Guide
Legendary (125G) Finish the Campaign on Legendary - (unlocks achievement for Normal and Heroic, if not earned).

Marathon Man (40G) Locate and access all Terminals in the Campaign.

The Ark

In the first building on the level, you will see a door almost immediately on the left across from a purple holographic panel. Simply enter this area of the level to find the first of the seven terminals.

When you go inside the structure to let the tanks across the bridge of light, there will be a Terminal almost directly behind the controls for the bridge. You must activate the bridge before you can access this terminal.

There is a part in the level in which you walk down a long hallway with white walls, and turn to the left at the end to continue. In the next room there will be a Grunts wandering around, and many stacked up plasma batteries. As soon as you enter this room, go underneath the staircase on which you are standing. Underneath there will be a door that conceals the terminal.

The Covenant

The fourth Terminal is located near the first elevator control. Before you ride the elevator up to find the Terminal start by looking at the access button. Next, look directly to the left you will see a dark ledge. Jump across to that, continue walking forward and the Terminal will open up.

After getting a Hornet vehicle, fly towards tower 2 (the one that the Elites had already disabled). A voice will remind you that Tower 2 is already clear, just to confirm you are at the right place. The Terminal is in the main hallway of the structure, across from the only door.

Terminal 6 is identical to Terminal 4, only in the 3rd tower and off to the right (versus the left in Terminal 4).

Halo

Terminal 7 is found at the beginning of the final level in Halo 3. To find it, head forward from the start of the level until you encounter the area with a few structural beams going from one wall into another. At this point, turn right, and look for a beam separate from the rest. Follow this beam into a doorway. Cortana will ask you, “Where are you going?”. Turn left once inside, and the Terminal should be in full view. Cortana will say "Wait. What's that?" as a good hinter. The terminal will be in front of one of Installation 04 II's believed pulse generators.

Meta Game (10G each for total of 90G)

Each level has a required number of points that must be earned. Turn on Gold skulls for bonus points, keep your amount of time playing in mind, don't betray any UNSC soldiers, and kill as many Covenant or Flood as possible. The method and number of kills is more important than anything else. Certain methods of kill earn more points than other. Most levels can be done on Heroic difficulty but Halo and Cortana may require Legendary due to how "small" the levels are.

Graduate (10G)

Earn 5 EXP or finish 10 games to complete the requirements for Basic Training (Online).

UNSC Spartan (15G)

Earn your Sergeant rating to be recognized as a true Spartan (Online-20 exp).

Spartan Officer (25G)

Advance to the Spartan Officer ranks (Online-70 exp and 10 skill). You may want to earn for experience points in Social matches in order to get better so your rating does not go down when you lose.

Two for One (5G)

Score a double kill with a single spartan laser shot in a ranked free for all playlist (Online). If you move as you fire the Spartan Laser, it can create a "fan effect" which can kill multiple opponents simultaneously.

Also, by "setting up" the achievement (meaning forcing other players to do a particular set of sequences), you can place the Ghost in the area where the Shotgun spawns on Snowbound. Allow a player to use it, then wait for someone to attempt to hijack it. Keep the Spartan Laser charged behind a shield door and grab the achievement.

Triple Kill (5G)

Kill 3 enemies within 4 seconds of one another in a ranked free for all playlist (Online). Can easily be done on objective games: Odd ball, King of the Hill, etc. Stand back and focus on killing instead of wining the objective.

Over Kill (5G)

Kill 4 enemies within 4 seconds of one another in a ranked free for all playlist (Online). Very hard unless you have cooperation or excellent skills. Best done on small map with swords or rockets. See * Two for One. Can be done easily in "King of the Hill", just wait outside the hill and finish off everyone inside.

Lee R. Wilson Memorial (5G)

Score 5 grenade sticks in any ranked free for all playlist (Online). Best done on swords with plasma grenades. Instead of killing with weapons, focus on sticking them only.

Killing Frenzy (5G)

Kill 10 enemies without dying in any ranked free for all playlist (Online). Use an objective game and grab a Sniper Rifle and pick off one person at a time. Kill and hide...someone may catch on and try to stop you. If you do this with a Sniper Rifle you can also get the Headshot Honcho achievement.

Steppin' Razor (5G)

Score a Triple Kill with the sword in a ranked free for all playlist (Online). Wait and watch for people to gather together in sword games or use a Bubble Shield to protect yourself and allow people to come at you and pick them off. * See Two for One.

Mongoose Mowdown (5G)

Splatter an enemy with the mongoose in a ranked free for all playlist (Online). Best done accidentally or play the entire game trying to run someone over instead of using weapons. Isolation and High Grounds are good hunting grounds. You'll need to be driving at top speed and hit your target squarely.

Up Close and Personal (5G)

Kill 5 enemies by melee or assassination in a ranked free for all playlist (Online). Best done in swords or with Gravity Hammers. Focus on beating down people instead of using a weapon.

MVP (5G)

Earn the MVP by scoring the most kills in any ranked playlist (Online). This you will really have to earn by scoring the most out of either team (losing or winning). .

Maybe Next time Buddy (5G)

Board the same vehicle within 10 seconds after being boarded in any free for all playlist (Online). Can be done on Rumble Pit. Sign on with guest and have the guest board you and then you board their vehicle.

Too Close to the Sun (5G)

Destroy an enemy Banshee with the Spartan Laser or Missle Pod in a ranked playlist or in campaign. An easy spot to complete this achievement is near the start of The Covenant after the first batch of Brutes and Grunts. Banshees occupy the airspace there, and Master Chief spawns with a Spartan Laser. You can easily shoot down a Banshee in the area that the Pelican drops a Mongoose.

Fear the Pink Mist (5G)

Kill 5 enemies with the Needler in a ranked free for all playlist or in campaign. Campaign on Normal first level.

Headshot Honcho (5G)

Kill 10 enemies with sniper headshots in a ranked free for all playlist or in campaign.

We're in for Some Chop (5G)

Destroy an enemy vehicle with equipment in a ranked playlist or in campaign. One way to get this achievement is to start Tsavo Highway second loading point, keep a fuel rod gun and wait for the Brute Choppers to come after you destroy the Wraith. You may have to kill all of the UNSC troops first as they tend to get a little greedy and destroy the choppers before you do. When the first chopper charges you, blast it 2-3 times with the fuel rod gun or shoot at the vehicle itself with a full round of Battle Rifle ammo, and throw your Trip Mine directly at the damaged Chopper so it blows up on contact, destroying the vehicle and its driver. Can be done on Normal.

Used Car Salesman (5G)

Destroy a vehicle that has three enemies in it in a ranked playlist or in campaign. In the hanger of Crow's Nest two Phantoms come inside the hanger, three if playing on Heroic and Legendary. You can also wait until after Forward Unto Dawn drops off the three Scorpions and the Gauss Warthog. There are multiple opportunities where a Phantom is stationary and in range. Using the Scorpion's primary weapon, roughly 5-7 direct hits will shoot it down. (Note- You can also sometimes use a plasma grenade on one of the dropships turrets. Sometimes, if your lucky, it will destroy the turret and get you the achievement.)

Skulls (10G each for total of 90G)

Find and grab each skull by pressing the right bumper (default controller settings), once you hold it for a few seconds, the achievement message will pop up and you can drop it by pressing to switch weapons or throw a grenade.

Gold Skulls:

Iron - If you die while playing solo, you will restart at the beginning of the level (you cannot revert to the last checkpoint at any time). If any player dies while playing co-operative, you will reset to the last checkpoint.

Black Eye - Your Energy Shield will not recharge until you perform a melee hit. The Energy Sword and Gravity Hammer's R-Trigger attacks count as a melee attack. Hijacking also counts as a melee attack.

Tough Luck - Enemies dodge grenades, needles, and vehicle assaults with a far greater success rate. However, plasma and spike grenade sticks are still possible at close range, and from behind.

Catch - All AI-controlled units, friend or foe, throw grenades more often, where applicable. The Catch skull has no effect on Flood, as they don't throw grenades. It should be noted that they will throw a unlimited amount of grenades at their enemies. Grunts have been seen throwing more than 4 in a row. The skull does not seem to increase the number of Kamikaze Grunts, however.

Fog - Your motion sensor is no longer visible.

Thunderstorm- All enemies (and Sangheili allies) are promoted to the next available rank. The number of shielded Flood combat forms is also increased. Elites that would have been Majors do not become Ultras, however, and stay the same rank. Basically, field promotions for everyone!

Famine - All weapons that are dropped by enemies or are scripted to spawn on the ground (e.g., near a downed Pelican or group of dead bodies) have halved or reduced ammunition or charge. However, weapons found in cases still have the regular amount of ammo.

Tilt - All energy shields (including Brutes' and excluding the Master Chief's) are made a lot stronger. Human projectile weapons have a tough time taking down Brute Shields.

Mythic - Each difficulty setting becomes the next hardest, e.g. easy=normal, normal=heroic, heroic=legendary, legendary=mythic.

Trivia

 * Unlocking at least 49 achievements and 1000 gamerpoints gives the Security helmet and katana body in multiplayer.
 * Getting 1000 gamerpoints unlocks the Katana body. Finding all 13 Skulls unlocks the Hayabusa head.
 * "Marathon Man" refers to Bungie's Marathon series, where interactive computer terminals were plentiful.
 * The name of the "Askar" achievement is actually in Bahasa Melayu (the formal language in Malaysia) and the meaning is "soldier". Also in Swahili (the official language of Kenya and Tanzania), the word "askari" can mean soldier or policeman. Similarly, iun Pashto/Pashtu/Pushtu/Pakhto (representative of dialectal differences in pronunciation) Asgar can mean soldier, or more generally a member of any armed force. (Pashto is one of two official languages of Afghanistan, and is also spoken in the tribal region of Northern Pakistan.)
 * Originally, there were 49 total achievements. 49 is equal to 7 times 7, which is another seven reference in the Halo series. There are 30 total new achievements, taking the total Halo 3 achievements count up to 79 with 7 + 9 = 16, and 1 + 6 = 7. With 59 achievements after AU2, 5 + 9 = 14 = 2 * 7.
 * Out of the original 49 achievements, the online-only achievements (except better luck next time buddy and MVP) were only obtainable in the playlists Lone Wolves and Head to Head. Occasionally Double EXP Weekend playlists do have Ranked FFA playlist(s) as well, like: Legendary Brawl. The new achievements added with AU2 are obtainable in both ranked and social playlists, providing it is on the required map(s).
 * Some players have been known to use the language glitch for the Halo 3 XBL FFA achievements, and this is considered boosting and frowned upon by most Halo gamers.
 * The UNSC Spartan Achievement, which you receive for reaching sergeant status online, has been known not to unlock for some people regardless of rank. This was later investigated by Bungie and was found that this is because their consoles are turned off before actually recieving the achievement. It should also be noted that, with the new EXP system, the achievement can be obtained through reaching UNSC Spartan level on a playlist. This still, unfortunately, will not unlock the UNSC Spartan pin on a player's global service record.
 * The "Vidmaster Challenges" were from Bungie's Marathon series, where players who wished to prove they were the absolute best had to complete insane challenges such as playing through the game without the use of health items, with only fists, on the highest difficulty setting. Those that succeeded made videos to mark their achievement, thus spawning the term "Vidmaster".