It will erase everything.
If you have been using the iphone as designed, tehn everything will be on your computer, and you can sync it back.
You should be importing all pics taken with the iphone to your comptuer regularly as you would with any digital camera.
You should have been syncing your iphone contacts with your computer from day one.
All music should be on your computer in itunes.
Have you failed to do these things?
The restore process does not restore everything - I had to select the ipone device on the left and then select the music and podcast screen and check the sync button. It was not checked. Dont know why. As for contacts, I had to use Verizon backup/transfer app to recover all my contacts. Lost notes, reminders, etc. Lost pics, but I am guessing one needs to use icloud to backup pics for restoring.
I had to a restore, recommeded by Apple to fix problems with IOS6 and wifi not working. The restore process was a complete waste of time. So dont rush into doing a restore before you know what really happens. Poor documentation doesnt help understanding.