The Apple Watch has two buttons on the right side. The button that rotates is called the Digital Crown, and the other is called simply the side button.
Reboot Tip: To force a frozen Apple Watch to reboot, press and hold down the Digital Crown and side button simultaneously until you see the Apple logo appear (take about 10 seconds).
Screenshot Tip: To take a screenshot of the Apple Watch’s display, press and hold down the side button, then press and release the Digital Crown. The screen will flash white, you’ll hear a camera shuttle sound, then the screenshot will appear in the Photo roll on the iPhone the watch is paired to.