The short version of What You Need To Know About iSync and Tiger is this: iSync will be gone, integrated into Tiger's internal "synchronization engine." But there's more to it than that, of course.
A lot of Tiger's synchronization features serve the purpose of selling .Mac, Apple's $99 bundle of Internet services. Which makes sense -- the more stuff that .Mac allows you to do, the more likely you are to buy it. But Tiger's sync improvements go way beyond .Mac.
The big story here is that Tiger's synchronization engine is, unlike iSync's, open to the rest of the world. Users of third-party address books, calendaring apps, and the like: rejoice! If the developer of your particular program adds support for Tiger's new synchronization engine, you will be able to sync your data with anything Tiger's new synchronization engine can talk to.
According to Apple, that description would cover the same sorts of things that iSync supported: .Mac, iPods, mobile phones, and PDAs. There's no clear word yet on if Tiger will support other sorts of sync types, or if the device side of the equation will also be open for extension by non-Apple developers. (Both would be a good idea. If I ran a Mac IT department I'd love to be able to set my Macs to synchronize certain settings back to a home-base server under my own control, for example. And I'd rather let two of my Macs at home sync with one another via our fast local network, rather than forcing them both to chatter with a far-off .Mac server.)
But even more importantly, synchronization is no longer just about bookmarks, address books, and calendars. Now applications can synchronize any sort of data they like. Let's say you're using a launcher-palette utility like James Thomson's excellent DragThing (okay, you can pick your favorite if you're not a DragThing user like I am). Now if your favorite launcher developer adopted Tiger's new synchronization engine, you might be able to set particular launcher docks or tabs or palettes to sync with .Mac. Then if you made a change to the contents of one of your docks or tabs or palettes, when you moved to another Mac and synchronized with .Mac, those changes would also appear on that copy of your-launcher-of-choice.
Or make your own example. Basically, if you've ever been frustrated because the settings of some of your favorite apps drift apart on different computers you use, this technology can put 'em back together. Apple applications in Tiger that will support this new sync technology include Address Book and iCal (duh), but also Desktop and Screen Saver, Exposé, and the Dock. Think about those last three for a second -- if you want, your Dock settings can follow you from Mac to Mac. That's pretty cool.
All of this appears to be controlled from the .Mac system preference panel, which offers four tabs. From the Sync tab you choose what applications you want to sync and how often that sync should occur.
Will this new sync engine solve every data-synchronization problem Mac users face? Probably not. But because Apple is opening synchronization up to third-party developers with Tiger, many more of those problems will get solved than have ever been solved with iSync.