The ban on tattoos
My sister-in-law recently posted on Facebook a picture of woman's arm bearing a black Jewish star surrounded by fire. The caption read: "My friend's Holocaust tattoo."
Aside from tattoos being a dehumanisation tool during the Holocaust and therefore a weird kind of commemoration, they are also forbidden in Jewish law.
The source for the ban is the verse: "You shall not make gashes in your flesh... or incise any marks on yourself" (Leviticus 19:28). Body mutilation was common among idolaters at the time. The rabbis debated whether the prohibition to permanently mark your skin was limited to tattoos containing God's name or the name of an idol.
However, the consensus is that no tattoos of any form are allowed (Tosafot on Talmud Gittin 20b). The human body is godly and should not be defaced. There is an urban legend that having a tattoo prevents burial in a Jewish cemetery but there is no halachic basis for this.