An air gun is a gun which uses gas to fire a projectile instead of burning gunpowder that an ordinary gun uses.
The oldest air guns used a bellows to compress (squeeze) air in a tube. When the gun was fired the air was released. The only way for the air to go was by pushing the pellet out.
A later invention was the spring piston, these can only fire one shot before the spring has to be reset. Newer guns use air or another gas in a small cartridge, these guns can fire many more shots before the cartridge needs to be changed or refilled.
The projectile for an air gun is usually a pellet. Some guns of this sort fire small round pellets call "bb"s. Some people think this means ball bearing, but it does not. The letters bb stood for a size of shot that was used to make shotgun cartridges, This was used in the first bb guns.
The most popular calibres (pellet sizes) for air weapons are 4 1⁄2 mm 5 1⁄2 (also called .177 to .22). In many countries there are strict rules that limit the sales of air guns so not everyone can have one.