Circumcision is a procedure that involves the removal of the foreskin of the penis after separating it from the glans. This is a common practice in certain religious groups. Even in the USA, most infants are circumcised soon after they are born- for medical reasons, not religious.
As for the difference between a circumcised penis and an uncircumcised one- once the penis is erect, the foreskin slides back from the head of the penis, and the only difference is some additional skin on the shaft that you might feel if you stroke the penis and move your hand up and down.
In the words of E.L. James, “steel encased in velvet.” I’m sorry about the cringy 50 Shades of Grey reference, but I hope you get the idea. The difference between an uncircumcised penis and a circumcised one is more obvious when the penis is flaccid, an uncircumcised penis will have the foreskin covering the glans.
Circumcised penises and uncircumcised penises get equally hard. The erectile tissue is not in the foreskin. There is no difference in the level of erection between an uncircumcised penis and a circumcised one. However, uncircumcised penises have one benefit over circumcised penises during penetration. The additional foreskin creates a glide during sex, the skin may even result in additional stimulation for the clitoris. The glide of an intact penis can result in better orgasms for women. The foreskin also helps in maintaining the lubrication during sex. Of course, circumcised men can still learn to glide, and use lubrication, to achieve the same effect.