Phimosis is a Greek word describing the nose band of a bridle, or a muzzle, and this is in fact a very good description of the appearance of the conditions.
I say "conditions" in the plural because phimosis is used to describe two conditions with unrelated causes and vastly differing effects.
Infant Phimosis is the normal and correct condition of male infants' foreskins being attached to the glans of the penis by connective tissue. This is a necessary protection for the infant glans and must not be pulled at hard enough to cause pain, as tearing the foreskin away from the glans can cause permanent scarring.
By the time a child is about ten years old this Infant Phimosis is likely to have disappeared completely. If the foreskin is still tightly attached to the glans after the onset of puberty it is advisable to see a health care professional.
Adult Phimosis, on the other hand, can be a problem, ranging from mild to extreme.
Adult Phimosis is usually caused by a phimotic band, or ring on the inner surface of the foreskin. This band does not allow the foreskin to stretch in its normal elastic fashion, but holds it to a more or less single sized opening.
In the most severe cases (called Full or "Pinhole" Phimosis) the foreskin is not retractable at all, covering all of the glans even when the penis is in full erection and leaving only a small gap for urine and semen to escape through. Boys with this type of phimosis (unsurprisingly) develop masturbatory methods which do not cause them pain and which are often different from the techniques practiced by boys with fully retractable foreskins and boys with no foreskins. When people with Pinhole Phimosis begin a sexual relationship most find they can manage this in a normal manner with little or no pain, although rougher sexual contact can be more painful than for a man with a fully mobile foreskin.
Pinhole Phimosis does offer some hygiene difficulties, which some men approach by holding the end of the foreskin closed and at the end of urination and allowing the sterile urine to wash out the enclosed space between glans and foreskin.
Partial Phimosis offers more problems at the onset of sexual activity. Boys with a foreskin which can retract somewhat also develop their own individual masturbation techniques and usually have no idea that the condition of their penis is significantly different from anyone else's until they attempt to have sex. If they and their partner are not sufficiently artificially lubricated the foreskin is likely to squash up and form a barrier, rather than retracting smoothly along the shaft of the penis. If the penis is large or the vagina it is attempting to enter is small to is also possible that the foreskin will be forecably retracted, and become stuck behind the coronal ridge - a paraphimosis.
Phimosis is treated in different ways.
Some people opt for surgery and choose either a full or a partial circumcision with dorsal slits being quite popular, and some people choose to rub a steroid cream into the foreskin in accompaniment to careful retractile techniques in masturbation over the course of about three months, encouraging the foreskin to stretch without tearing.