A mohel is a Jewish man who performs the ritual of brit milah on a baby boy eight days after he is born. (Unless the baby is not healthy enough to undergo the ritual, in which case it is postponed.) "Brit milah" means "covenant of circumcision" and hence "mohel" means "circumciser." Technically the baby's father is commanded to perform the circumcision, but since most dads are uncomfortable with this role and/or don't have the training to perform it safely, the mohel is designated as the father's surrogate.
A mohel is trained in the ritual of circumcision according to the precepts outlined in classical rabbinic texts. In modern times they also receive medical training and it is becoming increasingly common to find mohelim (plural of mohel) who are also doctors.
The Reform and Conservative movements of Judaism also certify women to fulfill this role. A "mohelet" is a female mohel and "mohelot" is the word for more than one mohelet.