Sync will automatically add any song that is on your library to your iPod every time you connect it to your computer. In addition to adding songs to your iPod, sync will also remove any song from your iPod that you deleted from your computer.
If your iPod is set up to automatically sync with your computer, you cannot manually add or remove songs from your iPod. When automatic sync is enabled, it gives iTunes complete control over your iPod library.
An iPod can only be synced to one computer at a time. If you try to connect your iPod to another computer it is not synced with, you will not be able to add any song from that computer's iTunes library to your iPod.
If you choose to sync your iPod with another computer, it will automatically delete every song from your iPod before syncing.
If you want precise song-by-song control of your iPod, you may want to disable the automatic sync feature.