Basically, circumcision is when the foreskin is removed from the penis. The foreskin is a piece of skin that covers the head of the penis (the glans) to protect it.
In some cultures and religions it is removed at childbirth, or at the start of puberty. There are no real advantages or disadvantages of being circumcised in my opinion, its just personal preference, or tradition.
Below i have posted some links to some penises with and without foreskins. Beware that they are real pictures. (And no, its not mine!
This is a picture of a circumcised penis: http://foreskinrestorationchat.info/frcfs/7.jpg
And this is an uncircumcised penis:
Hope this helps