Introduction to Air Guns

Types of air gun shooting
Plinking -  Tin cans and targets in the garden. Often an informal introduction to air gunning.
Target shooting -  Normally will involve joining a local club to compete against others of similar abilities.
Pest / Vermin control - High powered air guns are a quiet and effective way to control small pests including rats, rabbits, pigeons and squirrels.
Types of Airgun
Spring guns - The most common way to power an air gun. Spring guns are either operated by a break barrel system or use an underlever. Break barrel air guns are lighter but underlevers are thought to be more accurate as the barrel remains fixed at all times. With both mechanisms there is a certain degree of vibration and noise when the piston and spring are released.
Pre-charged guns - Seen as the next step forward in air gun design. Here the guns are powered by compressed air storred in a chamber attached to the gun. With a sound moderator these are much quieter than spring powered guns. Many pre-charged air guns also have magazines making them very quick to reload. You will also need to purchase a means to recharge the air gun, either  a pump or a diving bottle. 
The maximum legal limit for an air gun is 12 ft/lbs. An air gun at or about this power is effective for vermin control up to a range of 30 yards. For target purposes a gun does not need to be this powerful. 
The 2 main pellet calibres in the UK are .22 and .177. The .22 is a larger heavier pellet, this is the most popular calibre for pest control.
.177 is used for target shooting. It is a small, lighter pellet with a flatter trajectory.