What does Other mean in iTunes memory?

My 16GB iPhone 4S keeps running out of memory. I tried syncing it today only to find out that "Other" is taking up 7GB of memory. The other categories are Audio, Books, Photos, and Apps which do not take up nearly that much space. I tried clearing out a bunch of text messages, but it didn't help. What exactly is this "Other"? What can I delete in order to gain memory back?

Other stuff.

Other tends to be miscellaneous things that usually cane with the phone already and youll probably never see them but your phone probably needs them