Does syncing run both ways? Or just from computer to ipad?

The music on my ipad has somehow been lost on my computer. (I think it has to do with me saving my music to my itunes account sometime in the distant past. The purchased songs are still there but the ones I originally ripped from CDs are gone.)

I am hesitant to sync the ipad with my computer because I am afraid that will erase the music from the ipad. Will that happen?

How do I sync the two so that the music on the ipad is moved to the computer's itunes library?