Leviticus 19:28 states explicitly, "You shall not cut into your flesh for the dead, nor cut any marks on yourselves; I am the Lord." Ancient peoples of the Near East often cut into their skin and mutilated their bodies to demonstrate grief. They also cut into their skin and filled the incisions with indelible dyes, creating tattoos of the deities they worshipped. These practices were forbidden to Jews not only because they represented pagan worship, but also because they ran counter to the biblical prohibitions against spilling blood and mistreating man's G-d given body. Today, most Jews still hold that tattoos are prohibited, as they are an abuse of a body that is "on loan" from G-d, and not one's to alter permanently.
Sources: Kolatch, Alfred J. The Second Jewish Book of Why. Jonathan David Publishers, Inc.; Middle Village, New York, 1985.