If you have an iPod full of stuff, but it's not the stuff in your existing iTunes library, when you connect the iPod to your computer iTunes will remind you that this iPod is synced to another iTunes library. It will then offer you the option of wiping out the iPod and replacing it with the contents of the library you now have.
iTunes will NOT copy what's on your iPod back into your library, unless - UNLESS - it's songs that you bought from the iTunes Store. If you have songs on your iPod that you scanned off of CDs, you will have to use utilities like PodWorks
to achieve the goal.