What is ASG (part I)?

Because two days ago we had our weekly airsoft game (not a very succesful one, to be honest) I want to start today a series of post which will explain -- what is a ASG, Air Soft Gun.

First let's take a part of definition of airsoft from Wikipedia:

Airsoft is a term used interchangeably to refer to one or both of the following subjects:

• A military or law enforcement simulation sport in which players participate in mock combat with military-style mock weapons and tactics,
  
• 1:1 (or sometimes 'minis' or 3/4) scale models of real firearms, with a typical example firing 6 mm or 8 mm spherical, solid plastic BBs of usually 120, 200, or 250 milligrams, with propulsion for the projectile provided by manually compressed springs, electric motors, or compressed gas.

So... this explains what I mean when I write ASG. I don't exactly know why it is called airsoft -- probably because of two reasons: the bullets (BBs) are propelled by compressed air (in spring or electric guns) or gas (in CO2 / Green Gas guns), and the effects of hits are somehow "soft". Of course you'll have bruise in the point where bullet hits you, but the bruise will be the size of a mosquito bite. Of course without protective glasses you'll probably get eye damage with enough bad luck. BB marks on face are also not nice, that's because I wear a face mask. :)

How does the air get compressed and differences between the most popular gun designs (spring, gas, electric) -- next time, hopefully -- tomorrow.