ACTION: Any description of what's happening is called Action or a Stage Direction (it's also sometimes called Narration). Action text starts around 1½ inches from the left edge of the page, and should end around ¾ of an inch from the right edge. Never right-justify action text--or any text in a screenplay.

Writing good stage directions is an art in itself-- and a vital one to master if you want to write for the screen. Look here for a few do's and don'ts.