I’ll just lift the description off the Google Browser Sync page :P
Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings â€“ including bookmarks, history, persistent cookies, and saved passwords â€“ across your computers. It also allows you to restore open tabs and windows across different machines and browser sessions.
I don’t think I need to further expand on that. :P I have yet to try it out, but will do so shortly since I frequently switch between my desktop and notebook and could be a better option to my current setup of using Syncback to do the synchronisation/backups between the two for the browser side of things.
Google Spreadsheets on the other hand allows you to create, store and share spreadsheets. Currently still under Google Labs, it’s aimed more towards those who constantly send spreadsheets to and fro via email. You can apply to try it out here. A writeup on it can be found here.