The Apple Watch tells time. Crazy, I know. As to how that time will look is going to be largely up to you. Watch faces will more or less become the ringtones of smartwatches — colorful, fancy, sometimes gaudy, sometimes over-animated, and more likely than not something for people to make money with. It's the screen you'll probably see the most, and there are many ways you'll be able to personalize it.
Here are all the watch faces Apple pointed out today at its "Spring Forward" event, including some with custom widgets (stopwatch, calendar, date), and the classic Mickey Mouse watch face.