When you "sync" your iDevice to your computer it is actually multiple step process. The first step is backing up the dat on your iDevice. The other steps are syncing stuff between you iDevice and the computer. The "stuff" includes, contacts, calender, songs etc. From page 140 of Ipad Users Guide:
Restoring from a Backup
You can restore the settings, application data, and other information from a backup,
or use this feature to transfer these items to another iPad. Make sure your computer
has an Internet connection and that you’ve installed the latest version of iTunes from
www.apple.com/itunes.
Important: Restoring from a backup is not the same as restoring iPad from the
Summary pane in iTunes. Restoring from a backup doesn’t fully restore iPad software.
Also, restoring iPad from a backup restores all data in the backup, including data for
applications. If you choose an old backup, restoring it could replace the application
data with data that isn’t current. For more information, see “Resetting iPad” on
page 125.
