A sadist or severe pedophile
who acts on his/her urges and sexually abuses someone under the age of consent. Though abuse against young boys is often cited the typical case of child molestation occurs between a young girl and a male member of her family.
Still the molester can be familar or a stranger, male or female and the victim can be of either sex as well.
Lately "child molester" and "pedophile" have become the cheap insult dujour for flamers and other internet pests.
A lot of sexual practices are weird and gross, but child molesters are the only ones that are unforgivable.