From StrategyWiki, the free strategy guide and walkthrough wiki

• Hitscan: A weapon that acts instantly. There is no bullet travel time. The exception is when playing online; if for example there is a 0.2s lag on your connection, whatever is in your crosshair in 0.2s time is what will be hit.
• Mutator: One of a group of special settings that can be toggled on and off before a match occurs. Mutators for UT3 include:

• Big Head - your head grows larger with the amount of kills you have
• Instagib - all weapons replaced by instagib rifles, which have a primary fire like a shock rifle, but they kill anything with one hit
• SlowMo - slows the game down considerably

• Projectile weapon: A weapon that takes a little time to travel to hit its target. The rocket launcher is the most obvious example.