If you’ve used import / export to keep data from multiple sources consistent, you’ve probably experienced the problems it can cause—duplicate data and overwritten changes.

Let's say you want identical data in your Palm Pilot and your Yahoo! Address Book. While at home, you add John Doe to your Yahoo! Address Book and later you add Jane Smith to your Palm Pilot during a meeting. At the end of the day, you'd like to get the same information in both places.

So, you export your Palm Pilot data and import it into Yahoo! Address Book. You get Jane Smith in your Yahoo! Address Book but you also get duplicate entries for every other contact. To avoid this duplication problem, you could first delete all your Yahoo! Address Book data before importing the Palm Pilot data. But then, since it only existed in your Yahoo! Address Book, you would have lost John Doe's entry. Arrgh!

Synchronization solves these problems by taking changes to your data, regardless of where they were made, and distributing them to your selected PIM.