Wikipedia says: A cock ring is a ring that can be placed around a penis, usually at the base, primarily to slow the flow of blood from the erect penile tissue, thus maintaining an erection for a much longer period of time.
Cock rings can be worn around just the penis or both the penis and scrotum, or just the scrotum alone, though this is usually called a testicle cuff. Rings can be made of a variety of different materials, most commonly leather, rubber, or silicone, though nylon and metal are also used either as the main component or part of the closure.
Some models include a protruding clitoral stimulator, designed to stimulate the clitoris during sex. Others vibrate, either vibrating the ring itself, or in a popular 'Dolphin' variant by using two removable bullet vibrators to provide stimulation to both the testicles and clitoris. Some cock rings also have vibrators attached which can be worn to stimulate a partner during sexual intercourse, especially in the scrotum or perineum.