Drywall is a building material used in walls and ceilings of houses.  It's made of gypsum sandwiched between two sheets of thick paper.  Sheetrock is a brand of drywall. 

All my life, I've called it drywall because that's what my dad called it.  Some people would refer to it as Sheetrock, and it always threw me off.  But I just learned recently that I've been right all along.  I went to Home Depot to buy some drywall and noticed that there are other brands of it besides the Sheetrock brand.  Although I ended up buying the Sheetrock brand, I bought "drywall", not "Sheetrock".  It's like calling a tissue a Kleenex or a copy machine a Xerox.  I've written about this before. #products