OK, here's a few to start.
FPS- Feet per second, the standard velocity BBs are measured in. The standard measurements are approximately 280 for a stock Marui gun.
RPS- Rounds per second, referring to the firing rate.
AEG- Automatic Electric Gun, running off rechargeable batteries. Best company for this (internal and money wise) is arguably Tokyo Marui.
GBB- Gas Blow Back, usually pistol. Runs off gas, usually semi-automatic.
LPEG- Lower Power Electric Gun- For younger people, unplayable with older. For example, CYMA, WELL, Marui BOYS series
EBB- Electric Blow Back pistols, runs off 4 AAA batteries. Pitiful velocity, unskirmishable and nothing but a paper weight or wallhanger.
Springer- Spring Gun, Single Shot Only. You have to cock each time, velocities are very low.
There’s a few, any more?