What happens when you Back Up your iphone to itunes?

What happens when you "Back Up" your iphone to itunes?

It simply means that all of your data(Music, Pictures, Music, Apps, etc) is backed up to your computer and if you need to restore your phone, you can have a copy of all your apps and stuff.

When you back up your device it puts a copy of you stuff on your computer. You can make it to were it doesn't backup your apps, or music, or tv/moveies, or pictures. I don't back up my music much. When you sync what it does it copy your contacts, notes, cookies (web broswer), etc.

Does a backup erase your information?