The problem with all this smart fitness stuff is that the data always lives in individual silos. RunKeeper has your running data, Nike Fuel has other motion data, your weightlifting stats are stuck in Fitocracy, your weight is tracked by Withings.... Google will attempt to make them all play together nicely with Google Fit.
Just announced at Google I/O, with an SDK to come in a few weeks and a public rollout later this year, Google Fit will provide a set of common APIs that will allow apps to provide their data to other fitness apps.
As an example, Google showed a fitness app from Noom (a company that makes a number of individual fitness apps for weight loss, step counting, and carido training). It provided an integrated view of meals logged, steps taken, and even weight data from a Withings scale. The point is this: With your explicit permission, your fitness apps can share data with each other, giving you a more complete view of your health and activity.
If Google plans to have its own Google Fit app, it didn’t mention it. There are lots of partners on board, though. The current list includes Nike, Adidas, Asus, HTC, Intel, LG, Withings, Mio, Motorola, Noom, Runtastic, RunKeeper, Polar, and Basis.