Single-shot BB guns are typically the cheapest and most common. A single-shot design may incorporate a reservoir for extra rounds to be held, but it still only shoots a single shot. Between each shot, the shooter has to make a manual action to load the next round.
A semi-auto gun is a repeater that self-loads the next round. This means it fires one round per pull of the trigger. This design is quite common and very popular. These guns are quite fun and do allow you to shoot quite quickly.
Full-auto BB guns allow you to fire multiple rounds per pull of the trigger. The gun will fire as long as you pull the trigger and as long as you have ammunition. They spew BBs at a rapid rate and aren’t particularly useful, but these guns are just fun—tons of fun.