Its like paintball, but a bit different. The game styles are usually different.
The site I play at they usually run hour long games throughout the day based on a scenario read out beforehand. Each team has a certain amount of medics and when you got shot (only has to be 1 BB on any part of your body, stops arguments) you sit down and call medic. They have to get to you and tap you back in, or if one doesn't find you within about 5 mins you bleed out and you can get back up and go find one yourself.
FPS, as already stated, is Feet Per Second. Most UK sites have a limit of 328 FPS for every gun, though some have 350 for AEGs and 500 for bolt action sniper rifles.