Drywall is a construction material used to produce flat walls, ceilings and other surfaces. Drywall comes in sheets or panels, most commonly 4-by-8 feet. A sheet of drywall consists of a layer of compressed gypsum, essentially rock that's been crushed into powder, between two layers of thick paper. To build a wall with drywall, you nail or screw the panels to the studs, fill the seams and fastener holes with a cement-like product called joint compound, or "mud," and sand the whole thing smooth. Drywall installs much more easily and quickly than old-style plaster, which goes on wet and requires a great deal of skill to produce a flat surface.
Sheetrock is a brand name for drywall products made by the USG Corp., which grew out of the United States Gypsum Co., the source of the initials. The Sheetrock line includes standard drywall panels for interior use, which come in 1/4, 3/8 and 1/2-inch thicknesses. Sheetrock also offers an array of specialty products, including thicker, more fire-resistant panels; flexible panels for curved walls; foil-backed panels to prevent moisture penetration; and chemically treated panels that resist mold. In addition, USG markets joint compounds, drywall tools and some plaster products under the Sheetrock name.
The name "drywall" references the key difference between this product and plaster -- it starts out dry and flat, and doesn't require manual smoothing with a trowel, as does plaster.
When people make offhand references to "Sheetrock," they're usually just referring to drywall, not any specific brand , just like when people ask for "a Kleenex," they'll probably take whatever tissue you offer them. Drywall goes by other names, including "gypsum board," "plasterboard" and "wallboard," which is the generic term preferred by USG.
United States Gypsum developed the first widely used drywall products. Since that company "got there first," the name it chose was the one that stuck, that is the "Sheetrock Drywall System." The company introduced the system in the late 1930s, but it didn't really replace plaster on a large scale until the nationwide housing boom after World War II. As of 2011, USG said it supplied about 30 percent of the wallboard for the North American market, including about 12 percent in the United States.