Generally, the others category includes messages, cache, app data (if it's stored separate of the application itself), system files such as a downloaded but not yet installed delta fix from Settings, dislocated files and of course trash. In most cases, it's messages, application data and cached files. Often times it can be dislocated files such as the case where you strictly remember that you've synced over some media, but it doesn't show up in your Music.app or Videos.app, and are simply taking up space even though they're not accessible. See if it's the media being dislocated by checking to see if your iTunes library amount of songs matches that on your device's Music.app
I would recommend either backing up and restoring if possible, or, check out the app called iClean. To narrow it down a little, you can search "iClean Remove and Clean". It does run for several dollars, but it does remove any cache and excess/unnecessary data from your device, especially if the device has not seen a restore in ages (and even after a restore).