What's a Screenplay?

You always hear movie buffs talking about how well written a screenplay but you never really know what it is. I always thought it was as simple as writing a story but I was wrong, writing a screenplay is something very complicated and it involves the writer becoming in touch with many elements around him because the ultimate goal is to create a story that can come to life. The page I have linked to gives a detailed description of how to write a screenplay with subtopics like "What exactly is a script?,What makes a good story" and "A Series of Shots and Short Lines/Poetry/Lyrics" just to name a couple. This website was one of the very few that was not trying to sell me something in order for me to learn how to write a screenplay something I appreciated.