Well, there is not really that much to the health app itself. Basically all it does is collect data points of various things and can display graphs over time. It can be automatically collected from the M8 (steps, flight of stairs climbed, etc.), things collected from external devices (blood glucose testers, Apple Watch in the future), or entered manually, such as your weight, calories eaten, etc.
It also has an area to enter a number of medical facts about yourself (age, blood type, medications, etc.) that could be helpful in an emergency. This information is available from the emergency dialer section of the lock screen.
And Apple is also working with health insurance companies, hospitals, and clinics to transmit various information to your doctor (only with your permission, obviously).
So it's kind of bare right now, but will become more useful over time.