Oh, c'mon guys. He came here to ask a question.
Circumsicion is the removal of the foreskin from the penis. Hopefully you know what a foreskin is... the elastic like skin that covers the head of the penis. If you do NOT have this.. you were circumcised.
It is most often done on a newborn to *supposedly* help prevent future infection... though this has often been argued, and, in many cases, proven false provided the child is kept clean.
Circumcision is also performed on men and boys who develop chronic or severe infections of the foreskin.
Other men do it for mere cosmetic reasons.
If you have more questions, don't be afraid to ask!