Shotguns

A shotgun is defined as a smooth-bore gun (not being an air gun) which:

has a barrel not less than 24 inches(60.96cms) in length with a bore less than 2 inches (50.08cms) in diameter;
either has no magazine or a fixed magazine not holding more than two cartridges; and
is not a revolver gun.

There are two main types of shotgun:

Over and under

Barrels on top of each other. Generally a heavier gun to hold and carry, less recoil. The design of choice for most clay shooters.

Side by side

Er, barrels laid side by side. Lighter to hold and carry, but with a bigger kick. Previously favoured by game shooters because it's lighter to carry all morning, now considered old fashioned (you have to wear a flat cap when carrying one).

There also the semi-automatic shotguns which hold two cartridges in a magazine with a third in the barrel. Very rare to see anyone other than pigeon shooters using them.

Gauge/bore

"The number of lead balls the same diameter that weigh one pound". Most guns use 12 bore (or 12 gauge in American) but 20 bore is also popular; the guns are smaller and therefore lighter and with less kick, but with modern cartridges often reckoned as effective. See Wikipedia for an (American) explanation.

Safety catch

Most guns have one but some are automatic when the gun is closed, some have to be applied manually. The cause of the common cry of "can I have that one again, the safety's playing up".

Guns can be bought for as little as £100 (but go to a reputable dealer) or you can pay tens of thousands of pounds; reckon on starting at about £300-500 for a new one.