Flag

In Capture the Flag Multiplayer gametypes, the Flag is what 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 what weapon the player was using. The flag can't be holstered, but can be used as a 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 is an instant kill.

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

Halo 2
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 a skull.

Halo 3
The flag in Halo 3 is entirely different than that of the previous 2 games. The Halo 3 flag is more of a banner than a flag. The flag is held onto the pole by 2 parallel bars, perpendicular to the pole, and flaps little between them. The skull polecap 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 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 inadvertantly captures the Blue flag.