FPS means feet per second. The velocity of the BB as it leaves the barrel.

A Hop-Up or Spinup system puts an upwards spin on the BB as it goes down the barrel. This reduces the affects of gravity to increase the flight time and range of the BB.

Most often single shot with high capacity magazine. A spring compresses a cylinder when pulling back the slide or cocking handle. Upon pulling of the trigger the spring is released a burst of air hits the BB. Spring guns can be the most accurate and reliable of the airsoft guns.

Fully Automatic. When the trigger is depressed and held the gun will keep firing until the trigger is released.

Semi Automatic. It means, each pull the trigger the gun, will fire once.
 

AEG = Airsoft Electric Guns (Electric Rifle)
 AEP = Airsoft Electric Pistol (High Grade)
 EBB = Electric BB Guns
 GBB = Gas BB Airsoft Guns