This is one of the questions I was asked on another forum, so I figure I'd post it here to start it off!
They're actually for both the cock and balls - they encircle both at the root, and maintain an erection and heighten sensation by keeping the blood supply in the penis. They can also help to prolong sex and prevent premature ejaculation.
They come in a range of sizes, the most common being a diameter of 1.5 inches to 2.5 inches. This is including both the cock and balls, so one may have a small cock, but large balls, or vice versa, and wear a larger size.
There are many different materials used as well, the most common being a rubbery plastic that stretches in every direction, and is a one-size-fits-most size. Adjustable ones made of leather are also popular, often lasting much longer than the plastic ones (they degrade and break after time), and have multiple snaps to adjust the size. The leather ones are more easily removed during sex, as they can just be pulled off the base.
Some men wear them all the time, some wear them just for sex. Some men prefer to keep it on during orgasm, while others take it off just prior to orgasm (the force of orgasm may feel stronger this way).
Dangers are little to none with the plastic or leather rings - with metal rings, there is a risk of excessive engorgement. If the metal ring is too small, the blood going into the erection can't get back out after sex (regardless of ice, pulling on it, time, etc), and the cock stays hard too long (another type of this is priapism). If this happens, GO TO THE ER. If the penis stays hard for too long, permanent damage and possible loss of the penis could happen. Metal rings are very fun (some are weighted, to increase the feeling), as long as the right size is worn.
Questions? Comments? Clarification? Just ask!