Once you’ve launched the iMovie application, you’ll see there are three main areas on the interface: The Library Panel is located along the left side of the screen. At the top are your iPhoto, Event and Movie Libraries, where all your media is organized and stored, and at the bottom is your Content Library which contains Transitions, Titles, Maps/Backgrounds, iTunes, Sound Effects and Garage Band. 

Note that you can hide the Libraries Panel, by clicking on the Hide button located in the top left corner of the interface, to give you more room in your browser to work. If you want to see your libraries panel again, just click on the same button.