GarageBand/The Big Picture
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It's important to understand how each iLife app fits in with the other five. After all, part of iLife's allure is that everything is tightly integrated.
GarageBand is in charge of: 
- Making songs and podcasts.
- Scoring videos made in iMovie.
GarageBand provides: 
- iTunes with your own songs.
- iWeb with your own podcasts.
- iMovie with an application to score movies with.
GarageBand relies on: 
- iPhoto to provide photos for podcasts.
- iMovie to provide movies for scoring.