First what the foreskin is not: The foreskin is not a flap of skin on the end of the penis. This kind of terminology has been used to imply that the foreskin is something redundant with no real function, that can easily be removed with no consequence. This is not the case.
The foreskin is special in that it is not directly attached as most skin is. It is free to slide up and down the shaft of the penis with almost no friction. The foreskin can be thought of as a continuation of the tube of skin that covers the shaft of the penis, but much longer. This tube of skin is firmly attached only at the base of the penis and at the head of the penis. In between the points of attachment this tube of skin has the special property that easily slides on the shaft of the penis rather than being firmly connected as most skin is. The foreskin's length and the fact that it is unattached in the middle allows it to slide up and down the shaft of the penis and roll in on itself over the head of the penis (see diagram below).
The foreskin is long enough so that it continues down the shaft of the penis and rolls in on itself over the head of the penis. For those of you who are only familiar with circumcised penises, that means that an intact penis has two to three times as much skin that a circumcised penis.
A foreskin is long. An intact penis has two to three times the length of skin that a circumcised penis has. When flaccid, most of this length is taken up in the double fold of skin covering the head of the penis. When erect the foreskin can roll back to allow the penis to lengthen while still allowing the skin on the shaft of the penis to remain loose. The foreskin is often long enough to cover the head of the penis while erect.
The foreskin is extremely sensitive. It is filled with nerve endings called stretch receptors that fire when they are stretched, rolled, or massaged.
The foreskin has twelve known functions.
-to cover and bond with the synechia so as to permit the development of the mucosal surface of- -the glans and inner foreskin.
-to protect the infant's glans from feces and ammonia in diapers.
-to protect the glans penis from friction and abrasion throughout life.
-to keep the glans moisturized and soft with emollient oils.
-to lubricate the glans.
-to coat the glans with a waxy protective substance.
-to provide sufficient skin to cover an erection by unfolding.
-to provide an aid to masturbation and foreplay.
-to serve as an aid to penetration.
-to reduce friction and chafing during intercourse.
-to serve as erogenous tissue because of its rich supply of erogenous receptors.
-to contact and stimulate the G-spot of the female partner.