Look at links Stephan postet, especially http://code.google.com/intl/en/apis/gears/gears_faq.html
You have to do log of everithing user is doing while offline (relevant changes). Then when online send this log to server (use worker pool and HttpRequest) How offten and what to sync depends on data you're working with. I'm working on application that enables users to work with their private messages while offline. First I sync offline data and then request for "fingerprint", whether thera are any changes (new messages, messages deleted from another PC). If yes request this changes (JSON). Data that I'm displaying to user are only from Local database and syncing them every time user manually refresh the page or every 30sec (just new messages).
