When you first launch Clipstart, it will prompt you to choose to either "keep references" to added videos, or to copy them to a central location. You can change this setting later by choosing Library from the Window menu.
When keeping a reference to videos, Clipstart stores the file path to the video file in a file called Videos.db in /Users/your-user/Movies/Clipstart. Clipstart will also use this folder for importing from a camera or SD card.
When copying to a central location, Clipstart stores both the Videos.db file and all the videos and thumbnail previews in the location you specify. This is convenient if you have an external hard drive for storing videos.
Copying files is actually a two-step process. First, Clipstart adds a reference to the original videos to its database. Then, it starts copying the videos. After each video is copied, it's reference in the database is updated to reflect the new location.