By the way, you can also use loops, live recorded instruments a bar or two at a time.
It's just like a piece of a style, and you build the song. These are great, and there are many great loops out there. For drums and bass, Drums On Demand : http://drumsondemand.com/ are real user friendly and sounds great. You see, mp3 and wave opens a well of possibilities you don't have with midi alone.