Things you can do with the Apple Watch:

Track fitness and health using several baked-in, wrist-facing sensors
Get iPhone notifications. (iPhone is required for this watch)
Control your AppleTV and iTunes
Talk to other Apple Watches
Pick up your heart rate using LEDs and sensors
Pay for goods with your watch at numerous nationwide stores
Trigger actions or dictate messages via Siri
Change watch faces
Run a variety of functions, like maps for turn-by-turn navigation or a communicator tool that lets you connect directly with friends’ Apple watches
Display your favorite photos
Tweet from your wrist
See airline flight information
Unlock a hotel room door by waving the device (Starwood)
Run widget-like customizable bits of info showing things like calendar data and music info
Get notification vibrations using a so-called “taptic engine”