To view Storage information for your connected volumes, select About This Mac from the Apple menu, then click the More Info button in the window that appears. Next, click the Storage tab to display storage information

Sometimes the "Other" category in the About This Mac window may seem large. About This Mac uses this category to count any files that aren't recognized as one of the other listed file types (audio, movie, photos, apps, backups). Examples of files that may be calculated as "other" include:

• Items in the OS X folders such as the System folder and caches
• Personal information such as documents, contacts, calendar data
• App Plugins or extensions
• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package
• File types not recognized by Spotlight

Freeing up disk space

If you need to increase the amount of available disk space on your startup drive, examine the items in your home folder. Look in the Documents, Downloads or Desktop folders to see if there are any files you don't need anymore. Use these suggestions to reclaim disk space from these folders:

• Delete items you don't need, or have copies of elsewhere, by dragging them to the Trash.
  Once you're sure you don't need the items in the Trash, empty the trash to reclaim disk space. From the Finder, choose Finder > Empty Trash. 
• Move items to another drive, such as an external hard disk or flash drive.
• Compress items you don't use frequently. Delete the originals of these files and keep the compressed copy instead.

Be sure to only delete items you no longer need, or have copies of elsewhere. It's a good idea to create backups of important files in case you accidentally delete an important item later.

See this article for additional suggestions on how to increase disk space.

Safe Mode and disk space