* Sheetrock is a brand name for drywall, much like Kleenex is a brand name for tissue paper.

* Drywall is the generic term for the gypsum board panels used in interior walls and ceilings.

So, when someone says "sheet rock," they're referring to a specific type of drywall made by the company USG. However, "drywall" is the more general term and is used to refer to all types of gypsum board.