Identical to injection molding except that hot liquid plastic comes out of the barrel vertically, in a hollow molten tube or "parison". The 2 halves of the mold close on it and compressed air blows up the parison like a balloon, forcing it outward to conform to the inside shape of the mold. After cooling, a hollow part emerges.
Most commodity grade and engineering grade resins may be blow molded. However, the "melt index", or viscosity, of the plastic must be high to keep the parison from stretching too much prior to mold closure. The resin is in the form of pellets before processing.
Blowmold's are used in many outdoor decorations. This page contains pictures of just a sample of the different kinds that you can get. The earliest blowmolds were made in the 1950's and started off at about 4" tall. Today you can get them as tall as about 6' tall.