In the latest version of GarageBand, you can use other music apps at the App Store and record them in your GarageBand tracks via Audiobus ($4.99). You can record your real-time playing of an instrument, use a prerecorded loop, or choose to let GarageBand play the instrument while you select the keys. The cool thing about GarageBand for iOS is that you decide how involved you want to be with the actual music creation process. I was able to put together loops and create a great-sounding song without ever playing a note. Once you find something you like by instrument, genre, and other descriptors, just drag and drop the loop onto your track screen to add it to your song.
The track screen is also where you'll find GarageBand's prerecorded loops.
