In BattleTech, the Autocannon is a rapid-fire weapon that fires tons of stuff to the general direction of the enemy, often hitting the enemy too. Calibers range from 30 to 120 millimeters. It is one of the few slugthrowers that are actually deadly against BattleMechs.
The autocannons can do damage, as designated, in 5-point increments, from 5 to 20 points, and is one of the few weapons that are capable of creating such devastation (LRMs and MRMs have longer range though).
Only the backwaters of the Inner Sphere use normal autocannons. There are two specialized forms of autocannons, developed by the Clans: the Ultra and LB-X models.
The LB-X Autocannons are, typical to the Clan technology, more expensive but they're also lighter and generate less heat. The LB-X models are also capable of firing cluster munitions just as easily - this makes the autocannon act more like a shotgun or a flak cannon. The cluster shots are indeed quite deadly against VTOLs, and when used against BattleMechs the clusters are capable of hitting multiple 'Mech locations.
The Ultra Autocannons, also appearing in IS units, are more or less like your average IS crap autocannons, but there's one additional advantage: They're able to fire at double fire rate if so required. In that mode it eats twice as much ammo, generates twice as much heat, and get two possibilities to hit - and also has a slight thance that the whole weapon jams.
Annoyingly there are no LB-X Ultra autocannons, though. Still, Ultra AC/20 is pretty annoying at double rate. "That hit me in center torso for how much?"
Sources:
Bryan Nystul. BattleTech Master Rules, 1998. FASA Corporation, 1998. ISBN 1-55560-352-1.