Very simply, iMovie and iDVD are the consumer-level versions of FCP and DVD Studio Pro. They get to the goal in different ways.
Import a DV video (or create one using a variety of video clips, or digital photos for a nice slide show). Edit that video with a few simple tools. Add some audio, or edit the audio in place. Export to iDVD, where you can create a menued DVD that can be used in any DVD player. iDVD uses a canned approach with built-in themes to make it a simple matter to quickly create an easy-to-use DVD.
I'm a little simplistic, I guess, but that's what those two apps do.
If you are used to the full-blown apps, then you may not have much use for the consumer-level versions. But, they do quite a good job within the limits of the target audience (the new, or occasional user, as well as more experienced users that don't need the additional tools that your pre-level apps provide) - and, they're free with most any new Mac. What more could you ask!
