Google Fit uses the existing sensors in your Android device to monitor how much you walk and run. It also integrates with all major fitness apps such as Runtastic, Noom Coach, Runkeeper, Strava, MapMyRun, and Withings. Others are sure to follow. Using the accumulated information from all these different sources it provides you with a lush range of visualisations as to your overall health and fitness.
It should prove to be the missing piece of the puzzle in the overall health marketplace. Anyone who frequently exercises and uses a range of apps to monitor their progress knows how frustrating it is to have to update and check multiple apps. With Google Fit you should be able to aggregate all that information in to one place and get instant feedback.