My iTunes wants to delete everything!

Hi, please send help.

I recently downloaded some music (not from iTunes) and put it on my iTunes. Most of my music is on my iPod. When I went to sync up the ipod and itunes to get my recently downloaded music on my ipod, it asked if I wanted to delete everything on my ipod. I don't. I have music on my ipod that is not on my itunes. So if I clicked yes to delete everything on my ipod I would be deleting a vast majority of my music, which I don't want. I thought the point of sync was to have both the itunes library and the ipod have the same music on it, to, you know, be in sync. obviously apple doesn't know that meaning.

please, someone send help.