The sexual organs of the male are partly visible and partly hidden within the body. Each part has a medical name and there are lots of other names that people sometimes use.
One of the biggest concerns of many men is the size of their penis. There isn't a right size for a penis at any particular age. Penis size isn't directly related to the ability to bring pleasure to a woman during sexual intercourse. Normally the penis is soft. But when a man gets sexually aroused, blood flows into the penis at a faster rate than it flows out. This makes the penis larger and harder and it is called an erection. The average penis is between 5 and 6 inches long when erect.
Not all erections are caused by sexual excitement. Guys, especially younger guys, can get erections at the weirdest times. These are called spontaneous erections, meaning that they are completely random. This can be embarrassing. However, they go away fairly quickly if you ignore them. Many guys also get erections during their sleep and have a random orgasm in their sleep. This is called a "wet dream" and is completely normal.
Some men are circumcised meaning that the skin which covers the top of the penis (the foreskin) is cut back after birth. Some men are not. Guys who are not circumcised need to pull back the foreskin when they pee or wash themselves.
The scrotum is the loose sack of skin at the base of the penis, in which the testes are hidden. Depending on how cold or warm it is, whether the guy is sexually aroused or not, the scrotum can shrink or expand in size.
There are two testes (sometimes called "balls") that are inside of the scrotum. The two testes grow in size and become able to produce sperm and male hormones like testosterone. It is best for the testes to be a slightly cooler temperature than the inside of the guy's body. They are very sensitive to injury. When a guy ejaculates, sperm that was made in the testes mixes with other fluids that are made inside the body and spurts out of the penis.Doctors recommend that guys do a Testicular Self Exam once a month to make sure everything is okay. The links below tell you how to do this and what to do if something doesn't seem quite right:
The seminal vesicles produce fluid that mixes with the semen before it is ejaculated.
The main function of the prostate is to store and release a fluid that makes up about 25-30% of the semen. This fluid combines with the sperm and seminal vesicle fluid. Prostate fluid helps sperm survive and live longer inside the vagina. The prostate contains some smooth muscles that help to pump the semen out of the penis during ejaculation.
There is more helpful information about a man's sexual body parts at the following links.