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iMovie is a consumer based non-linear movie editor whereas iDVD is a consumer-based DVD authoring environment. so you take the raw video clips, edit them into a movie in iMovie, then take that iMovie and make it into a DVD with menus, etc. in iDVD.

(PS: i went to syracuse for a few years back in the day. go orange!)
 
You capture and edit video footage in iMovie HD. Adding music, effects transitions etc. You can then export your edited footage into a number of formats.

iDVD takes previously created content from iMovie or iPhoto and allows you to organise, format and burn that content onto a DVD disk.

Two programs that mostly get used together, but with two different functions.
 
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