According to About.com, Sheetrock is a brand and is also a type of drywall; therefore, there is no difference between the two. By definition, drywall is "large, flat sheets of pressed gypsum that compose the inner walls of most residential buildings."
Drywall is one of the most basic materials used in house construction. The sheets typically come in 4 feet by 8 feet or 4 feet by 10 feet sizes and are between 3/8 and 1/2 inch thick. Some drywall sheets come in sizes up to 16 feet, but those can be difficult to transport.
There are also many different kinds of drywall. For example, ultra-light drywall is much lighter than typical drywall and is easier to transport and hang.