And what are these?
HealthKit is an application programming interface (API) built to help iPhone developers access and manage the user’s health and fitness. This takes the form of data like heart rate, exercise, and level of activity. Your iPhone, when paired with an Apple Watch, will be able to monitor you with surprising detail.
…the Health app will be available only on iPhone. This may seem strange, but Apple has restricted apps to only its phone or tablet before. The company feels Health would be of little use on the iPad because it rarely leaves home.
Matt gets into understandable detail here: iOS 8 Turns Your iPhone Into A Personal Healthcare Monitor.