^The idea behind airsoft is that you can shoot people and not really harm them, much akin to paintball; however, airsoft guns use mechanisms that can fit inside the weapon, thus adding an aspect of realism that paintball fails to achieve, along with the fact that the effective range is doubled or even tripled in comparison. The drawback is that airsoft is an honor based sport, since it leaves no visible marks unless at close range, meaning if you're playing with some dickheaded teenagers (or more often, dickheaded 30 year olds lol) people don't necessarily 'call their hits'.
Classic airsoft guns were usually spring or electric powered (using a motor and gears which pulled back a plunger in a tube which created the force when released; technically spring and AEGs are both spring powered, only the electric one does it automatically while spring required manual cocking. Spring died out years ago as a viable method. Gas is quickly replacing AEGs, as they've become more efficient and cheaper to produce. Gas can more easily incorporate blow-back mechanisms which further simulate real firearms, but generally requires more maintenance and can be more expensive in the long run.
When I was younger Glock trademarks were still legal somehow. I had a gas blow back G23 made by KJW, it was so real I actually couldn't tell the difference between firing a real Glock and firing the airsoft version, save for the harshness of the recoil and of course the muzzle flash/bang.
In classic AEGs and springers, the strength of the spring determined the velocity of the projectile (along with weight of the BB, I found anything less than .23 BBs weren't very accurate and would pay the extra dough for the quality). Generally 350 fps was a cutoff at most fields I played at, 300 fps for indoors. In japan they're required by law to stay below 250 if I recall. Every time I bought a Jap gun like a Marui I'd open her up and fit it with a 350fps spring. Marui made the best guns but needed some extra umph for us Americans lol.
I've found that past about 400fps you'll get welts/bruises and even bleed if you're closer than 50 feet, so I've never been a fan and find it sadistic/childish to use springs any stiffer than that, and even then 400+fps shouldn't be used at ranges closer than 100ft IMO.