Wallboard is a catch-all term for any material that's attached to studs to create finished wall surfaces. Drywall is a specific kind of wallboard made with gypsum.
By far, the most popular wallboard for interior use is drywall. Water-resistant cement board holds up better in wet areas. Other wallboard materials include fiberboard, plywood and even plastic panels.
Drywall is made up of a layer of gypsum, a soft mineral, sandwiched between two layers of heavy paper. It's often referred to as "Sheetrock," though that is a brand name.
Wallboard materials come in 4-by-8-foot panels. Since wall studs are usually placed 16 inches apart, a sheet of wallboard can span exactly three stud cavities if placed vertically or six stud cavities horizontally, minimizing the need for cutting.
Wallboard of any kind goes up much faster than traditional plaster walls and needs to be finished with plaster or joint compound only at the seams where panels meet and at nail and screw heads.
Drywall's name comes from its relatively short drying time, compared with plaster. Finished drywall can be painted within days. Plaster walls can take weeks or months to fully dry.