Capture the flag is a game where every team has a flag which they must defend and which other teams will try to take (or capture). To capture a flag you must first have to move over another team's flag holder. After that, you will have the flag in your hand, but because of that, you cannot shoot any of your weapons (except heavies; heavy units will still be able to fire a pistol while holding a flag). However, the flag still makes a great melee weapon. If you use it properly, you can kill any units with only one hit with the flag (even heavies). After that, you just have to move back into your base and over your flag holder to "take" the flag.
The game ends when the timer runs out or a team captures three flags (from any team or teams) before the timer runs out. When the timer runs out and no team has captured three flags, the team with the most captured flags wins.
Question: Is it worth trying to capture the flag rather than just killing other teams?
The answer is yes. When you capture the flag, you get almost one hundred points for capturing the flag! (Points received in games with score requirements are increased by 10%).
Question: I have another team's flag in my hand and am over my team's flag holder, but for some reason I can't capture their flag!
You cannot capture another team's flag if another team has your team's flag in their hands. In order to receive your flag back, you must either wait for them to capture it or for the player holding your team's flag to be killed (you will know once the server says, "So-and-so has dropped Team So-and-so's flag!).
Capture the flag and the Minimap
In a CTF game, you can see all of your teammates from any distance on your minimap.
See also: Minimap