Bezel
The bezel is the ring that holds the crystal and frames the dial. It can be fixed, or it can rotate to measure dive time (a one-way diver's bezel), a second time zone (GMT), or speed (tachymeter).
A bezel is more than a decorative ring; depending on its type it has a job. On dive watches it rotates one way only: you line the minute marker up with the hand before a dive to read elapsed time, and because it turns one way, an accidental knock can only shorten the time, never dangerously extend it. A GMT bezel uses a 24-hour scale to show a second time zone. A tachymeter bezel works out average speed over a set distance. Fixed bezels simply frame the dial.