The penis is the male copulatory organ, and, in mammals, the male organ of urinary excretion. It is homologous (develops from the same embryonic structure) as the female clitoris.
The penis is built of three columns of erectile tissue:
The end of corpus spongiosum is enlarged and cone-shaped and forms so called glans penis.
Erection is achieved by influx of blood into a labyrinth of blood vessels. The vessels are distended and reflex muscle contraction around veins traps blood in vessels of erectile tissue. (When this mechanism fails to function properly, the result is impotence.)
The urethra that is the last part of urinary tract traverses the corpus spongiosum and its end lies on the tip of the glans penis. Apart from being a passage for urine it also is for semen ejaculation.
Many societies have practiced circumcision on religious or cultural grounds. A Brit milah (Hebrew) or Bris (Yiddish) is a Jewish ritual circumcision required by Jewish law. Most Muslims circumcise their children as a religious duty as well.
In the modern USA, many male infants are commonly circumcised on the grounds that circumcision reduces the risk of certain cancers and sexually transmitted diseases. In recent years, modern studies have again proven that such health benefits exist, but the amount of protection provided turned out to be lower than expected and previously believed. Given that there are certain health risks involved in the procedure, routine circumcision of all male children is no longer reccomended by most doctors.