The scrotum is the loose pouch of skin containing the testicles (or balls). The primary function of the scrotum is to protect the testicles, which are very sensitive to pain, and which also need to keep an even body temperature to function properly. The testicles serve two main functions: producing sperm and producing testosterone. For the sperm to work there needs to be an even temperature. The scrotum helps with this by either keeping the testicles tight against the body when its cold (the famous shrinkage effect when you get out of the water) or hanging loose in hot weather.
Inside the testicles, the epidymis is a coiled tube where sperm mature until they travel through the vans deferens before they are ejaculated during ejaculation.
There is a great difference in the size and look of testicles from one man to another. Most (but not all) men have two testicles; it is not uncommon for one to hang lower than the other.
Both the scrotum and testicles can be part of sex play and sexual pleasure. A lot of men enjoy gentle (or firm) stimulation of the testicles, which can involve licking, tugging, or simply cradling the scrotum.