There are two types. One goes just around the penis and this one is used to help maintain an erection because it restricts blood flow out of the penis. Believe it or not but this type is (or can be) an approved medical device. I have a friend who has one that was prescribed by a urologist for his ED.

Then there are those that fall into the sex toy catagory. These are generally worn around both the penis and the scrotum for cosmetic or recreational reasons. They make the dick and balls look larger both when nude and under clothing. My balls ride high and when I get an erection they pull up even higher and all but disappear. A cock ring holds them down in my scrotum for a nicer appearance. And even when I'm soft the cock ring provides me with a larger, fuller contour in a form fitting Speedo-type bathing suit or in jeans or other snug fitting pants.

I tried both metal (even metal that was claimed to be surgical steel) and the silicon/rubber types and they all caused a skin rash. So I use a child size jade bracelet. Works like a charm. I've worn it 24/7 for more than ten years now without any ill effects. Yes, even during medical appointments. Doctors have seen everything and nothing shocks them. Trust me on this. The jade bracelet has the added advantage of not setting off metal detectors in airport security lines.