What exactly is circumcision and why do they do it?

Removal of foreskin. The foreskin is a piece of skin that covers the glans, or the head of the penis. In circumcision, this foreskin is removed. There is no medical necessity for this, but it is associated with a lower incidence of urinary tract infections, and penile cancer. Often it is done due to tradition or for religious reasons.
Removal of foreskin. Mostly done in infancy because parents think it is cleaner and will avoid infections. May need to be done if foreskin is tight (phimosis) and doesn't retract easily & does not respond to steroid cream. Some adolescents want to look like the majority of other boys. It does prevent penile cancer which in itself is very rare and more likely caused by poor hygiene. Less smell. Some think looks better.
Prevention. Circumcision is the surgical removal of the foreskin, a small flap of skin that covers the tip of the penis, generally performed in the days after birth. Examination of scientific studies between 1995 and now shows links between circumcision and decreased risks for urinary tract infections, some kinds of cancer, hpv, hiv, and other sexually transmitted diseases. There is a preventative effect.