Bezel
The bezel is the ring that holds the crystal and frames the dial. It may be fixed, or it may rotate to measure dive time (a unidirectional diver's bezel), a second time zone (GMT) or speed (tachymeter).
A bezel is more than a decorative ring; depending on its type it does a job. On dive watches it turns one way only: you align the minute marker with the hand before a dive to read elapsed time, and because it turns in just one direction, 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 calculates average speed over a set distance. Fixed bezels simply frame the dial.