The iPhone is synced with another iTunes library.

My iphone 5 was originally synced with another computer. I wanted to add a tone to the phone and got the

The iPhone “name” is synced with another iTunes library on “name”. Do you want to erase this iPhone and sync with this iTunes library?

I'm not concerned about any music getting lost, but what exactly gets erased if you sync with a new library? Is the whole phone apps, contacts etc erased?

any help is appreciated!!