What is Dungeons and Dragons?

From what I’ve heard, it seems like a lot of fun. The problem is, I haven’t heard a lot. What is dungeons and dragons, a kind of board game, or what? I’m not sure if there are a bunch of versions or whatever, but I’d like to play the original first if there are. Just any general info would be nice, and a link to somewhere where I can order a set (Is it a set?). Thanks for the help!