What does the game Warcraft have to do with World of Warcraft? Also World of Warcraft expansions?

I am looking into getting WoW. But I saw something about a game called Warcraft that seems to be some kind of prequel. Please tell me what it has to do with the game. Also about he WoW expansions? Do they just unlock new places to go in the game? New characters? Please any additional information I may need is greatly appreciated!