Circumcision most often refers to male circumcision, in which the foreskin is cut off. It is a comparative minor 'surgery', although quite memorable to the person it is done to. It is often preformed without anesthesia, although in the mid-1900s is gained popularity as a medical procedure for newborns, and nowadays when a circumcision is preformed in the hospital it is common to use local anesthesia. However, most people do no longer believe that the medical benefit of circumcision (better penile hygiene and decreased chance of passing STDs) is worth the trouble of preforming surgery on newborns. A notable exception is male children of the Jewish faith, who are ritually circumcised soon after birth in the brit milah ceremony.
Circumcision is not by any means a Jewish invention. Hundreds of cultures worldwide do it, and have done it for thousands of years. I cannot speak for any individual's experience, but it seems that in its proper place it is not a bad thing, culturally speaking.
Circumcision is usually a juvenile rite of passage, making a boy a man, warrior, or at least a young adult. It can be very important for the person it happens to. It can also hang on as a pointless and negative tradition. It may represent the removal of the mother's blood from the young man, giving him completely over to the men. It might also be a test of bravery, or the symbolic death of the young person and the birth of the adult, or just a experience all male members of a society go through -- A symbol of unity and membership. Circumcising young children (under 10 years of age) is a comparatively rare idea. Most circumcisions (by number of current and past cultures doing it) are done around the age of puberty.
There is also female circumcision, a much more serious operation where the labia or clitoris is removed (or sometimes both). It can represent a symbolic childbirth or just serve to get rid of the 'male' shapped thing down there, making a woman all female. Female circumcision is pretty much always a Bad Thing; while it has largely the same symbolism as a male circumcision, it is much more dangerous (and painful), and may result in serious infection and long term complications. You can find much more on female circumcision in that node.