All men who joined the Schutzstaffel (SS) had their blood group tattooed under their left arm. It allowed medics to quickly discern the blood group of a wounded SS man for transfusions, even without ID.
Early in the war, the tattoo was made in Fraktur gothic lettering but subsequently changed to latin lettering. The tattoo was about 7 mm in length and placed on the underside of the left arm, about 20 cm up from the elbow.
Although the tattoo was officially mandatory for all SS men, not all actually had it. SS officers often avoided getting it, probably because they didn't want to mark themselves as members of the hated organization. Foreign SS volunteers were also often not tattooed.
After the war, having the tattoo made all members of the SS marked men. Allies checked POW's for the tattoo and placed any men found to have it in special POW camps that only housed former SS. Soviet troops rampaging through Germany often checked the underarms of men to see if they were members of the SS. Chances were they shot any SS men right away.
When Adolf Eichmann was arrested in Argentina, the Mossad agents (most of whom were concentration camp survivors) checked his underearm for the tattoo, as he had been a member, but he had had it removed.
Sources:
http://www.skalman.nu/third-reich/ss-blood-group.htm
http://www.wssob.com/insignia.html
Kudos to Cletus for assistance.