What the “Other” Storage Space is in Mac OS X?
Essentially it’s anything that OS X does not allocate to the listed specified storage types of applications, backups, audio, movies, backups, and photos. That means a very broad list of items will be considered as Other, including things like the following:


Documents and file types, including PDF, doc, PSD, etc
Archives and disk images, including zips, dmg, iso, etc
Various types of personal and user data
Anything in the system folders of OS X, ranging from temporary files, swap, voices, etc
User library items like Application Support, iCloud files, screen savers, etc
User caches and system caches, including things like browser caches and locally stored message media files
Fonts, app accessories, application plugins, and app extensions
Various file and file types not recognized by Spotlight, for example a virtual machine hard drive, Windows Boot Camp partitions, etc


As you can see, this is not unnecessary junk or clutter. Basically, anything that is not one of the media types that the Storage tab specifies will be shown as “Other”.

Source: OS X Daily: What the “Other” Storage Space is in Mac OS X &amp; How to Clear It Up