We've known Google Fit was coming for a while now, and there have even been some tantalizing leaks, but now the official app has arrived. Google Fit is a hub for all the fitness data being fed into Google's platform. It's compatible with just about any device under the sun, and looks pretty nice. It'll look best on Lollipop, though.
The changelog says it best—under what's new, Google says "Everything!" The Fit app will display data from any third-party service that uses the Fit API, which includes the Fit app on Android Wear devices. Fit will display walking, running, and cycling activity (a few more options might be nice). All the data gets bundled together in nifty graphs and charts that tell you how you're doing.
Google Fit plugs into your existing Google account, allowing you to add information like height and weight. The activity tracking and notifications are also optional, so you can still manually add activity to Fit without having it always active. Note, it looks like this update will wipe step data on connected Android Wear devices. It also seems like Fit data sync across devices is pretty wonky and slow. Might simply be a hiccup.
Update: The Fit website is live too.