Timeline is a new way of presenting yourself on Facebook...
Gone is the single column wall of posts you're used to for profile updates. Gone are the neatly divided panels of every Facebook profile page that existed up until now. Gone is the struggle to figure out what someone you've met only recently was like a few years ago. Gone is the static feeling that all your past updates have.
In your old profile's place is a page that gives over much of its real estate to updates presented down the page as a dual-columned array of tiles that contains status updates, links, photos, media and a whole lot more. And oh yeah, you can plaster a giant-ass panoramic image across the top of your profile.
...that dives deep into your past...
The Facebook wall has always been structured chronologically, but has become less and less favorable to browsing posts and updates from more than a few months ago. In timeline, that all changes. There is now a literal timeline embedded on your profile that will let anyone viewing it click back to any year where there are updates or content and view them. As someone goes back further in time on your profile, fewer updates appear; eventually, only major events and ones that you've specifically picked for inclusion are highlighted. But fear not, you can also remove any particularly embarrassing updates from your timeline.
...but also makes your history dynamic...
Was there a major event from your past that you didn't care to talk about then, but wish people knew about now? Timeline will allow you to go and create new events or add new photos for things that happened years back. Zuck and Co. want Facebook to be less of a time capsule and more of a curated biography. But it's not just about changing the past; Timeline is all about highlighting recurring themes and patterns and connections with other people in your life, thanks to Facebook's Open Graph app framework.
...while giving apps a new lease on life...
Zuckerberg admitted that their first attempt at giving users freedom with apps was kind of a failed experiment. In the days of Facebook modules, people could create heavy, bloated pages full of nonsense. Facebook eventually took that functionality away, banishing apps to the homepage sidebar. But now they're bringing 'em back. Operating in an app sandbox and making use of Facebook's Open Graph framework, users can share updates, media and interactive features from the apps, which appear seamlessly alongside all other updates and posts. And it delivers information with context. It shows the songs you've listened to most in the last month, meals you've cooked in the last week, or your favorite books over the past year.
...and works just as well on mobile as it does the desktop.
Despite the slick look of the new UI, it wasn't designed solely for the traditional browser. It works just as well on mobile devices, shapeshifting to fit the new screen size. Is this their rumored HTML5 site at work? Perhaps, although Facebook hasn't officially said yet. What we do know is that Facebook isn't scared to experiment with notions of what their site is, and what it can be.
P.S. It's coming sooner than you think.
Zuckerberg says that developers can start signing up for early access now, and over the next few weeks Facebook will start rolling Timeline out to us commonfolk. Keep your eyes peeled!