Click on the Finder Face in your dock.
That will open a window with all your User files/folders.
Select the one named Movies.
Within that are 2 more folders iMovie Projects and iMovie Events.
Events are the raw unedited footage in iMovie.
NOTE WELL: iMovie Projects are only small text files with instructions on where to find all the events, music etc. and how to put it all together.
If you delete your events, your projects will not work.
If you want to delete your events, you must first export your projects (Share > Export Movie..).
That renders your movie and sets all in place.
You can at any stage re-import that exported movie back into iMovie as a new event and do more editing.
So, do not trash your events until you have rendered your projects.
After that you can trash both the projects and the events and just keep the exported movies.
Best advice: Buy a 1 or 2TB external drive with firewire connection.
You can then keep all your video on the external drive and not clutter up your HD as well as having a Time Machine backup for your computer.
Z.