Wikipedia
Sicherheitsdienst , full title Sicherheitsdienst des Reichsführers-SS , or SD, was the intelligence agency of the SS and the Nazi Party in Nazi Germany. The organization was the first Nazi Party intelligence organization to be established and was considered a sister organization with the Gestapo, which the SS had infiltrated heavily after 1934. Between 1933 and 1939, the SD was administered as an independent SS office, after which it was transferred to the authority of the Reich Main Security Office (Reichssicherheitshauptamt; RSHA), as one of its seven departments/offices. Its first director, Reinhard Heydrich, intended for the SD to bring every single individual within the Third Reich's reach under "continuous supervision."
Following Germany's defeat in World War II, the SD was declared a criminal organisation at the Nuremberg Trials, along with the rest of Heydrich's RSHA (including the Gestapo) both individually and as branches of the SS in the collective. Heydrich's successor, Ernst Kaltenbrunner, was sentenced to death for war crimes at the Nuremberg Tribunals and hanged in 1946.