Flag

A flag is a piece of cloth, often flown from a flagpole and emblazoned with a design used to represent a faction.

Rebels
Human rebel forces possess several flags throughout the various insurrection movements. Of the many flags, some depict insignia such as family crests, bloodied dragons, and scorched crossed swords.

Multiplayer
In Capture the Flag Multiplayer gametypes, the flag is an item that must be taken, ran back to your base, and "captured." Halo's CTF is much like the real-life game, except that you actually have a real flag on a flag pole, which is usually unheard of in the real world. Also, rather than hiding the flag, it is always in a pre-determind position, often in the middle of the base, and your only method of defending it is to kill other people and wait for them to respawn.

While carrying the flag, it temporary replaces the previous weapon the player was using. The flag can't be holstered, but can be used as a very efficient melee weapon. Depending on the rules of the variant, the flag can deal much more damage than a normal melee would. In the default settings for CTF in Halo 3, a flag melee can cause an instant kill.

History
The flag in Halo: Combat Evolved was very basic, with very little ripple animation or pole design.

Halo 2 saw a number of upgrades to the flag. The design and shape of the flag is still the same, but the flag pole got a major redesign. In Halo 2, the Flagpole is capped by Captain Jacob Keyes skull.

The flag in Halo 3 is entirely different than that of the previous two games. The Halo 3 flag is more of a banner than a flag. The flag is held onto the pole by two parallel bars, perpendicular to the pole, and flaps little between them. The skull pole cap is still there. The emblems have changed, now they bear a symbol of the UNSC, and the only defining characteristics are their colors.

Trivia

 * On Bungie.net, the description of the multiplayer flag states that "It's right next to the headlight fluid." This is an obvious reference to Red vs Blue, in which Donut is tasked with finding elbow grease and headlight fluid and inadvertently captures the Blue flag.
 * In Halo 3, if you stick an opponent with a Plasma Grenade or Spike Grenade, then picked up a flag before the grenade exploded, the kill will count as a Flag Kill.
 * A glitch in Halo 3 sometimes makes the cloth of the flag fall off of the pole, thereby "breaking" the flag.