How is a subwoofer connect to a 2 channel system?

Since I've never ventured into higher end systems than my current one that uses a multi-channel receiver, I'm interested to find out how people hook up subwoofers in a 2 channel system. I know subwoofer often have speaker level inputs and there are subwoofer specific amplifiers available, but I don't know much more than that. I see 2 channel systems running dual subwoofers and wonder how they are setup. This is just a fact finding mission to increase my knowledge.