Some hard drive space gets lost to formatting the drive platters.
The OS uses up space.
If you haven't emptied the Trash icon in the OS X Dock, then anything in there is still eating up space on the hard drive because you haven't emptied the trash to remove/erase that data.
Applications you install eat up hard drive space.
If you use Apple Mail a lot, then you can amass a pretty good amount of data in emails, especially if you save emails over a long period of time.
If you have any other things like if you use iTunes and have a massive amount of audio data,then that eats up hard drive space.
Also, the Movies portion of that readout is about both any purchased, commercial movies, as well as any "finished" or "finalized" movies you have created with iMovie or Final Cut.
If you kept all your original iMovie file data, then ALL of that gets saved in iMovie project folders that take up additional hard drive space and, also, under the Others category.
It is not really all that hard to figure out, Anything data you create, download, install, saved to a computer's hard drive is going to use up valuable hard drive space.
iTunes, iPhoto, and, in your case, iMovie or Final Cut can eat up large amounts of hard drive storage space.
Do a web search for, download and install OmniDiskSweeper.
http://www.omnigroup.com/more
This app can tell you what and where data is being taken up on your Mac's hard drive.