Nowdays there are three main varieties of pistol: "automatic" self-loading pistols, and revolvers, being by far the two most common types, followed distantly by single-shot hunting or target pistols. Pedantically, the chamber wherein a pistol's charge is ignited, is fixed in relationship to its barrel -- thus the term technically excludes revolvers, although in colloquial usage this distinction is commonly ignored, and revolvers are quite commonly, albeit informally, referred to as "pistols".
Revolvers feed ammunition via the rotation of a cartridge-charged cylinder, which incorporates multiple ignition chambers, sequentially brought into alignment with the weapon's barrel by means of the manual manipulation of the weapon's trigger (double-action), or of its hammer (single-action). These nominally cylindrical chambers, most typically numbering six, are situated around the inner circumference of the rotating cylinder in such manner that their axes are parallel to the cylinder's axis of rotation; thus, as the cylinder rotates, the chambers revolve about the cylinder's axis.
"Automatic" pistols use the recoil or gas energy of each round to cycle the action, extract the spent case, and load the next cartridge. Automatic pistols are more accurately semi automatic, in that each pull of the trigger fires a single bullet; however, there are a number of fully-automatic pistols such as the Glock 18 and later models of the Mauser C96. Single-shot pistols are loaded manually via the breech, either from a small magazine or by hand.