The abbreviation for the infamous Schutz-Staffel (German for "Protective Corps")
The SS an elite para-military unit of the German Nazi party. They were formed from the ranks of the SA in 1925 to be Adolf Hitler's personal guard. By the time the Nazis came to power in 1933 the SS had over 50,000 members called "black shirts" (from the black uniform they wore).
From 1929 the leader of the SS was Heinrich Himmler, who together with his right hand Reinhard Heydrich consolidated the power of the organisation. By the time World War II began the number of members rose to 250,000 and the Waffen-SS was formed as an elite unit of the Wehrmacht, as Germany's armed forces were then called. During World War II the SS was responsible for the running of concentration camps.
The SS had their own secret service, the SD (Sicherheitsdienst), which served as a political police force.
After the war, the SS was declared to be a criminal organisation responsible for the executions of POWs, resistance fighters, Jews, Poles, Gypsies, and all other political opponents.