If you check the contents of your Mac's hard drive (or flash storage), you might see a certain amount described as "Other."
All of your audio is accounted for distinctly. So is the space used to store apps, photos, movies, even backup files that get queued up when your Mac is away from its Time Capsule or Time Machine backup drive. So what's "Other?"
I have 15.51GB of other, and 8.31GB of movies on my MacBook Air. I have no idea what so ever what 'other' is. I also have no movies on my Mac. I've deleted all messages, all emails and there's nothing in the Downloads.
"Other" is a catchall OS X uses to describe a whole bunch of stuff stored on your Mac's hard disk. The operating system files and any caches or logs that are created is included in that list, for example. Any app plug-ins (for Safari and Firefox, for example) or extensions that you've installed go in that category. Any files or documents you've created are also included in "Other." Media files that are dependent on other things — media files stored inside of application packages, say — are included in "Other." So are any additional filetype that Spotlight can't identify.
The amount of space you use with Other is going to vary dramatically depending on how many third-party apps you have installed, how many files and documents you've created and saved to your disk, and how large the cache files are. OS X handles those cache files as part of its routine housekeeping, so you shouldn't feel the need to do anything with them yourself.
On my Mac, I have almost 80 GB worth of files that are accounted for as "Other." So your 15 GB is looking pretty good to me!