Testicles, which hang just below the penis, are the dominant sex organs in males. They are round in shape and tend to be slightly smaller than the size of golf balls. Their shape has spurred some of the more commonly used slang terms for testicles, such as: balls, family jewels and nuts.
Testicles play a major role in reproduction since this is where sperm is produced. In simpler terms, they are equivalent to the ovaries in women. In fact, in fetal development, the gonads for males and females (which later develop into sex organs) are the same up until around six weeks of age. The gonads will become either testicles or ovaries during the first trimester of pregnancy.