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It's not an app, exactly. It's more of a hub where you can monitor daily health stats, and see them over longer periods of time.
It's not the first we've heard of Apple pushing into healthcare - rumors have been flying since earlier this year - but now we finally have a few concrete details on what the app will look like and how it'll work.
HealthKit will talk to all of your fitness apps, including third-party apps, to keep track of your health monitoring in one place. It can also read your vitals, and notify your doctor if something is amiss.
The Mayo Clinic is among the several major hospitals and institutions that will work with Apple and support HealthKit.
Samsung just announced something similar last week, called SAMI. It will be interesting to see how the two services compare.