There's no stupid questions from those who ask nicely
Airguns, also called pellet guns, usually use either CO2 powerlet or compressed air (pump or break barrel) and generaly shoot lead .177 pellets, copper BBs, between 350 and 600 fps. They are not safe *at all* to shoot at anything that live
They are used for target shooting mainly, and are usually quite accurate but single shot
They usually don't try to mimic the look or functionality of real firearms
Airsoft guns come in a lot of different look and feel, usually are 1:1 "replica" of firearms (or so close it would look identical to the untrained eyes), the most common ones being modeled after the Ar-15/M4/M16 assault rifles.
They shoot plastic 6mm BBs, and can be considered "safe" to shoot at people *under the right circumstances*. That said, they always should be treated as weapons and not like toys. They *are* weapons and care must be taken when operating them.
Velocities usually varies between 300 and 500 fps, but the projectile being much lighter (.2g to .43g, as opposed to more than 1g) than airguns', the energy carried over impact is much less than the later.
They are much more complex mechanically and are more prone to failure and less accurate than pellet guns. There are two kind of propulsion means:
electric, where a system of gears are put in motion by an electric motor in the grip will compress a spring, on which end is a piston. At the end of a cycle the sping is released, piston compresses air in the cylinder, propellin the BB, and then the cycle start over; this allows for semi and automatic fire (about 10 to 50 rps are possible)
The other system mimic the real firearms mechanism, and are gas operated. A gas (usually propane) is store in the magazine, and released in the gun's chamber where it expands and propell the bb. A valve will redirdect some of the gas and will push the bolt backward (causing the gun to recoil) and spring will push the bolt back into battery and load a new BB in the chamber, ready to fire next round.
Airsoft guns are usually more expensive, less reliable and less acurate than pellet guns, but can be used in the sport of the same name (Airsoft), similar to what Paintball is.
That pretty much sums it up
Hope it clarifies things for you!