The categories change as new versions of the OS are released, but the current categories include System files, Documents, Apps, iTunes, Backups, Other, Audio, Music Creation, Photos, Movies, and Purgeable space.
Mouse over a color bar, and a text balloon will pop up, telling you what the category is and how much disk space it’s using. If you’re not sure how to find the categories, click the Apple logo in the menu bar in the Finder, select About This Mac from the dropdown menu, and then click the Storage tab.
Originally introduced in OS X Lion as part of a feature upgrade to the System Information app, the Storage tab displays how much free space is available on each disk attached to your Mac, as well as how a disk’s storage is being used, by category.
One of the niceties in the Mac OS is the ability of the system to display how your storage space is being used.
