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Quake Live Walkthrough Game Modes - Capture the Flag
Again, a bit self-explanatory. But there are some things you need to know.
Depending on what side you're in and when in the game you're in, you'll either be defending your flag from enemies or attacking the flag.
Staying near the flag would be a good way to defend it.
I find staying near the exits from your base pretty helpful too.
Also, just going out looking for people to kill is a good thing to do.
After your team gets a certain amount of captures, they will win and sides will switch.
Ammo and power ups and such are around the map also but you only have a weak machine gun and the guns that are available to you on the map.