"C&B play" stands for cock & ball play. "Genitorture" stands for "genital torture". This is a subject that makes some men clutch their nuts and run in fear, and makes other men instantly erect and greedy for more. The male genitals are at once the most vulnerable and most sensitive part of the male body, so of course many tops enjoy playing with them.
Cockrings are rings that go around your cock, typically around the base of it, behind the balls. The penis becomes erect when the blood vessels at its base constrict (because of arousal), trapping blood in the cock and causing it to swell. Cock rings have a similar effect, prolonging erection in most men that use them. (They also constrict the urethra, which will make any orgasm more painful, or even cause ejaculate to back up into the bladder. This is not dangerous unless done repeatedly. Experiment to find out how much tightness is too much.)
Most are made of leather, with adjustable snaps, so you can tighten or loosen them to fit (as well as remove them easily). Some are made of rubber. Some are even made of metal, but metal ones can be hazardous; if you put a too-small one on your non-erect cock, your cock may become so erect that you can no longer remove it--and if it is too tight, it will prevent your cock from softening. This may involve a trip to the emergency room and the use of bolt cutters. No joke.
Some cock rings have multiple rings, for behind the balls, around the balls themselves, and around the base of the shaft. Some people like using _lots_ of cock rings, to stretch the balls out away from the body.
Safety tips: The broadest guideline is to go slowly until you know how much you can take. If the pain from a particular activity starts to spread into other areas of the body, or if the pain lasts for a long time after the stimulation ends, you have probably gone beyond your limits. You won't reach this point generally if you take your time. As with any SM practice, if you find yourself in pain later, or if you notice any abnormalities in your cock or balls when flaccid or erect, see a doctor. Of course, avoid any practice that seriously wrenches or twists the genitals; there are many ligaments and blood vessels in there, damage to which may make it hard for you to get hard. But the cock and balls can handle light whipping or slapping, provided it is done with care.
Of course, cock and ball bondage can be done with leather strips, ribbons, velvet cords, etc. Be as ornamental as you please; tying up an erect cock can create a luscious work of art, and teasing it can be even more artistic. Don't expect C&B bondage to keep a cock hard indefinitely; cocks will usually get soft if not stimulated, and bondage which _will_ keep it hard may be dangerously tight. In any event, be sure you can remove your bondage quickly, as always.