From Halopedia, the Halo wiki
Medals are preset rewards in multiplayer or in the campaign that showcase special achievements that a player makes through the game, usually by killing opponents in a special way, a certain number of opponents at a time or in one life, or by achieving some objective. Medals appear in all Halo games except Halo: Combat Evolved and Halo Wars.
Halo: Combat Evolved
Multiplayer medals do not appear in their physical form in Halo: Combat Evolved, but some of their accomplishments are occasionally listed despite this. The Announcer will announce Double Kills, Triple Kills, Killtaculars, as well as Killing Sprees and Running Riots. However, none of these medals have physical representations, and Xbox Live has no way of recording them as the game does not utilize Xbox Live.
Campaign medals, awarded for completing certain actions within the campaign, were originally intended to appear in the game, but were ultimately cut before release. They would have been more similar to Achievements rather than later campaign medals.
Medals made their first real appearance in Halo 2. There are 23 medals available to win, which fit into four general categories; "Consecutive kills" are the streak of medals which acknowledge a number of kills within four second of each other. Double Kills for two, Triple Kills for three, Killtaculars for four, Kill Frenzies for five, Killtrocities for six, and Killmanjaros for seven kills. "Spree" medals are awarded for a number of kills a player achieves without dying. Killing Sprees are given for five kills in one life, Running Riots for ten, Rampages for fifteen, Berserkers for twenty, and Overkills for twenty five. The third Category is "special kills" and these are awarded on the basis of special ways a player kills another player. There is an award for killing players with Sniper Rifles, Plasma Grenades, vehicles, and melee attacks from the front and back. There is also a medal for making Carjackings.
Also, there are objective medals available in special gametypes. The CTF gametype has a medal for taking a flag, killing a person who has a flag, and returning a flag to your base. The Assault gametype has a medal for when a player plants a bomb and when they kill someone holding the bomb.
Most of these medals can be viewed in-game when they are earned, and also after the game in the post-game carnage report. Prior to the termination of Xbox LIVE support for the original Xbox console on April 15, 2010, Bungie kept track of the games played and medals earned, though there was no official record for players to keep track of how many medals they have earned throughout their careers. Because of this, sites such as Halo2junk which kept extensive records of player profiles and medals earned grew in popularity. Following the discontinuation of original Xbox online support, Halo 2 service records, both official and third-party, ceased to be updated.
With the release and announcement of Halo 3, medals was given a significant overhaul, more so than Halo 2. Most of those featured in Halo 2 reappeared in their original form, while many others have undergone a complete overhaul, and still more have been created. 42 Medals were introduced in the Halo 3 Beta and 56 were included in the final game. Medals were created for new gametypes such as VIP and to acknowledge special forms of kills, such as the kill with an Oddball or a Flag. Others are for scoring a point in an objective based game. Specialized spree kill medals were made to acknowledge kills with certain weapons, such as sniper rifles, shotguns and energy swords. In addition, special medals were created to be displayed only on Bungie.net showcasing certain achievements.
A Medal Chest was created on Bungie.net for Xbox LIVE accounts. The chest avidly kept track of all medals a player has won throughout their matchmaking and campaign career. On March 31, 2012 Bungie.net ceased updating Halo game stats and player records, transferring all record-keeping responsibilities to Halo Waypoint. Though players can still view their statistics compiled prior to that date, their Halo 3 service record is no longer updated.
Halo 3: ODST
With Halo 3 having multiplayer and campaign medals, medals were put into Halo 3: ODST in both the campaign and Firefight Modes. All medals that were in Halo 3, except the wheelman medal have been carried over into ODST, and a few Firefight specific medals added. The Multi Kill Medal has also been removed from campaign in favor of using the same kill spree medals used in multiplayer. Receiving medals gives a boost in points that one would not receive if they were to just kill the enemy normally.
The Medal Chest available on Bungie.net also has a section for Halo 3: ODST medals. The chest shows all medals the player has won throughout their Firefight and campaign career. On March 31, 2012 Bungie.net ceased updating Halo game stats and player records, transferring all record-keeping responsibilities to Halo Waypoint. Though players can still view their statistics compiled prior to that date, their Halo 3: ODST service record is no longer updated.
More medals can be seen in Halo: Reach than all the previous games combined. In the final game, there are 112 different medals.
Medals return in Halo 4 with a more emblem-like appearance compared to the realistic medal appearance in Halo: Reach.
Halo: The Master Chief Collection
The Master Chief Collection features the most extensive suite of medals yet, including nearly every medal awarded in the Halo series to date.
Halo 5: Guardians
Halo: Spartan Assault
Medals in Halo: Spartan Assault feature a simplified blue monochrome appearance.
Halo: Spartan Strike
Medals in Halo: Spartan Strike are identical to those in Spartan Assault though they are cyan rather than blue.