Here's another take:
Apple got slammed hard in the beginning of the iTunes Music Store for not carrying a lot of popular, independent labels. Now, they've added hundreds of songs from independent labels much to the fanfare of the users. Good move. What's next? Well, the logical step would be to make it easier for independent artists to sequence, edit and package their music -- enter GarageBand. For $50, artists with a relatively inexpensive computer setup can now record, mix, arrange, and lightly edit their music and export it as AAC files for placement on the iTMS.
SoundTrack is more for what the name implys -- soundtracks are usually musical scores that go along with some sort of video production, whether it be a song played as action occurs on the screen or some sort of ambient music that needs to sync up to on-screen action. Sure, it's great for dabbling around with and making cute little songs out of the supplied loops, but the capabilities of SoundTrack go far beyond making pre-packaged music. GarageBand takes over here for "the rest of us." You can get thousands of loops for $100 so you can really be a one-man band (you play guitar, and drop in some pre-made drum loops, etc.).
If an analogy must be made, it could be that GarageBand does for music what iMovie does for video, the same that SoundTrack does for music what Final Cut Pro does for video -- just on a much more advanced and "pro" level (hehe... like what brianleahy said!).