What is "other" space on my iphone 4s and how do i get rid of it?

So I went to sync my iphone 4s today and I noticed that "other" space was taking up 13 gb of my memory! i only have a 16 gb phone! what the heck. i cannot seem to figure out what is taking up all this space.. What takes up all this space and how do I free it up?? HELP!

thanks!