The Apple Watch comes with a robust array of fitness-tracking features, including three separate rings tracking movement. The Move ring will tell you if you've done a normal amount of brisk human movement for the day, while the Exercise ring will let you know when you've completed a workout. The Stand ring will measure how long you stand during the day and will doubtless make bloggers feel extremely bad about themselves.
The Watch will give you customised reminders to reach fitness goals, and comes with its own dedicated Workout app, which can measure calories, time spent sweating, and a variety of other rubrics. It will be able to tell if you're running or biking. It will be able to gently chide you for doing so more slowly than you planned. It will share that data with the HealthKit app.
I wonder if Fitbit execs have started sobbing yet. The Apple Watch looks like it's going to be a serious contender in the fitness tracking market.