FPS stands for Feet Per Second, for those of you who hate anything other than SI units (Systeme Internationale), one fps is 0.304 meters/sec, (equivalently 1m/s = 3.28 fps). This is why 328fps is thrown around as a limit. 328fps is 100m/s, and a 0.2g bb travelling at this speed possess the energy of exactly 1 Joule.
Energy (in joules) = 1/2 mass * velovity^2
FPS is the most common way of measuring the velocity of an airsoft gun. The average FPS rating is 250 for lower end guns and 300-500 for higher end guns. Sniper rifles vary from 350 up to 500 fps.
The average AEG shoots about 290+ FPS, with the average MPEG shoots about 300 FPS, the average LPEG shoots under 220 FPS, and minis usually shoot 115 FPS. Springers are various. Small spring pistols seldom get above 250, while snipers, as stated above, shoot up to 500. Gas powered guns usually have higher FPS than other guns their type, but use a different propulsion method. 145 FPS is about 100 miles per hour.
The FPS listed on websites selling airsoft guns is the FPS measured with a standard .2g bb. Heavier bbs will decrease the FPS, but increase the amount of kinetic energy the bb has in flight, thus increasing the hitting power. There is no known sure formula for how FPS is effected by BB weight, so it must simply be tested in each gun.
FPS is measured by Chronographs, which are similar to a police officer's radar guns, in the way that they measure the speed of an object moving past it. These can be bought for as low a sixty dollars.
UK FPS Limits
In the UK the standard FPS limit for an AEG or GBB Rifle is 350fps with snipers coming in at 500 fps with a minimum engagment range of about 25 meters. These do vary from site to site but this is the genral rule of thumb.