Summing up what others have said and providing a bit of context:
Studs are vertical structural components in the walls. In load-bearing walls, they carry part of the weight of the house and its contents above them. In American construction this generally means a "2x4" stud placed every 16 inches on center along the length of the wall, with additional support at critical points and "cripple studs" to carry those forces around window and door openings.
Non-loadbearing walls are generally constructed ("framed") the same way for consistency.
The surface of the walls is then attached to this framing. There are several different ways of applying that surface -- it could be decorative wood panelling, for example -- but one of the most popular interior surfaces is painted plaster.
In the past, constructing a plaster-surfaced wall required nailing up thin boards (lath) to create a supporting surface, then trowelling plaster over that. Some of the plaster would be squeezed between the lath, giving the plaster a good grip on the wall. After several cycles of plastering, sanding the surface smooth, and plastering again this built up a smooth, fairly strong, fire-resistant wall surface ready to be primed and painted.
Wallboard/plasterboard/drywall is a modern shortcut for achieving the same effect. Pre-cast sheets of paper-backed plaster can be attached directly to the studs with screws. That leaves only the joints between adjacent sheets to be plastered and sanded by hand.
Note that since the studs at on 18" centers, 4 or 8 feet is a good size for getting complete coverage of the wall with minimal cutting of the drywall sheets. That same spacing is used elsewhere in the house's construction, so "sheet goods" generally (plywood, panelling, waterproof "green board" for bathroom walls) will tend to come in that 4'x8' size, or occasionally 4'x4'.
Hope that gives you a bit more understanding of how "stick framed" walls work. If you want to know more about lumber-framed house construction I'd really recommend picking up a book on the topic. There's just too much interesting and important detail to cover completely here.