Male circumcision is the removal of some or all of the foreskin (prepuce) from the penis.
Jewish law states that circumcision is a 'mitzva aseh ("positive commandment" to perform an act) and is obligatory for Jewish-born males and some Jewish male converts. It is usually performed by a mohel on the eighth day after birth in a ceremony called a Brit milah (or Bris milah, colloquially simply bris), which means "Covenant of circumcision" in Hebrew. It is considered of such religious importance that the body of an uncircumcised Jewish male will sometimes be circumcised before burial.
Rationales for Jewish Circumcision
from Moses Maimonides, "The Guide of the Perplexed", Part III, ch. 49:
"With regard to circumcision, one of the reasons for it is, in my opinion, the wish to bring about a decrease in sexual intercourse and a weakening of the organ in question, so that this activity be diminished and the organ be in as quiet a state as possible. It has been thought that circumcision perfects what is defective congenitally. This gave the possibility to everyone to raise an objection and to say: How can natural things be defective so that they need to be perfected from outside, all the more because we know how useful the foreskin is for that member? In fact this commandment has not been prescribed with a view to perfecting what is defective congenitally, but to perfecting what is defective morally.
The bodily pain caused to that member is the real purpose of circumcision. None of the activities necessary for the preservation of the individual is harmed thereby, nor is procreation rendered impossible, but violent concupiscience and lust that goes beyond what is needed are diminished. The fact that circumcision weakens the faculty of sexual excitement and sometimes perhaps diminishes the pleasure is indubitable. For if at birth this member has been made to bleed and has had its covering taken away from it, it must indubitably be weakened." - Ben Maimon