Basically, it synchronises your saved passwords, browsing history, and open tabs between different machines with Sync installed. If you use Firefox Mobile (née Fennec), you can open a tab from your desktop machine on your mobile device, and vice-versa. Mozilla have even announced an iPhone client that (presumably) launches Safari instead of Firefox, but still gives you access to your open tabs and history:
https://mozillalabs.com/sync/
Basically, it synchronises your saved passwords, browsing history, and open tabs between different machines with Sync installed. If you use Firefox Mobile (née Fennec), you can open a tab from your desktop machine on your mobile device, and vice-versa. Mozilla have even announced an iPhone client that (presumably) launches Safari instead of Firefox, but still gives you access to your open tabs and history:
http://blog.mozilla.com/blog/2010/05/26/firefox-home-coming-...