Fitness and Workout apps track movement, activity, heart rate, intensity and more. Apple highlights the quality, frequency and quantity of movement. Apple wants to encourage people to become more active, whether you are serious or light exerciser currently.
Apps shows how many breaks you take, and much more. You can also set goals, in terms of distance, calories or other statistics. There is also a companion app for the iPhone, which is separate to the Health app. It shows much of the same statistics in a broader view as the mobile Fitness app on the watch itself.
The Workout app seems to be the more serious of the two apps, where you can set specific goals for specific activities. The Fitness app is more general, and meant for the more casual fitness individual.