There are two main programs that I use for editing video files, too. Final Cut is definitely the better program of the two, but is more difficult to learn how to use at first. If you're looking for a simple, easy editing program, iMovie is for you.
Final Cut Pro
Final Cut Pro is an Apple program. It is probably one of the best video editing programs out there, providing non-linear editing of any QuickTime compatible video format. It supports a limitless number of simultaneously composited video tracks, up to 99 audio tracks, and features much more. Although it is perhaps a bit confusing for beginners, once you learn how to use it, it's a wonderful resource.
iMovie
iMove is also an Apple program included in the iLife package. Although you can't do as much with it as Final Cut Pro, it's pretty easy to use right away. The biggest problem I find with the latest version of iMovie, however, is that there is no save option. You simply have to keep backing up your files until you have finished editing and can compress and export the entire thing.
