There are so many of these products on the market that it's almost impossible to pick just one for review. Most are very stretchy and made of latex or silicone.
Cock rings are multi-purpose devices. Initially developed to aid in prolonging or enhancing an erection, they've evolved to include clitoral stimulation -- either via a nubby extension and/or a bullet vibe.
Check out what Babeland has to say about using cock rings: http://www.babeland.com/sexinfo/howto/use-a-cock-ring