From Watches Wiki
Metal ring with a device in which the watch is inserted to protect the hands and the dial. The bezel is an important part of the watch case. This ring can be fixed with the case or be turnable.
In dive watches the bezel is provided with minute markers, and is designed turnable in one direction (counterclockwise) (turning bezel) to set the dive time; other sports watches sometimes are equipped with a bezel with tachymeter.