Most revolvers are handguns, but some shoulder-fired firearms are also revolvers. On those, the barrel is longer and a shoulder stock is supplied.
The vast majority of revolvers are not pistols, because the chambers contained within the cylinder are separate from the gun's barrel. Those revolvers that do contain multiple barrels in the cylinder are most often referred to as pepperboxes rather than revolvers.
A revolver's cylinder contains multiple chambers, which may vary in number. The most common number is six, and this is why many revolvers are called six-shooters - but many guns have been built with varying numbers of chambers. I have handled revolvers having five, six, seven, and nine chambers, and other variations are in existence.
Revolvers are usually of centerfire or percussion type, but Elisha Collier's patented flintlock revolver did see some success in the early 1800s.