When you import video, or record directly into iMovie, iMovie stores that video (your raw footage) as one or more Events in the Event Library. This is the source video for all your video projects; when you create a movie or a trailer, you select video clips from your Events and add them to your project. You can then edit the video however you want within the project without modifying the source video. As a result, after editing video in a project, you can always revert back to the original, unmodified state of the source video.
When you import video, you name your Events, and then by default iMovie lists them in the Event Library by the date they were recorded. You can choose to organize the library in other ways, and you can also hide the library to give yourself more room to work.
In addition to being a powerful tool for making movie projects, iMovie is useful as a video library, a platform for organizing, accessing, and watching all your footage. By giving your Events meaningful names, such as “Birthday Party 2010,” “Family Vacation in Belize,” and so on, you can organize all of your video so that it’s readily accessible and then watch it right in iMovie.