Adolf Hitler founded the Schutzstaffel (SS) in April of 1925, as a group of personal bodyguards. As time went on, this small band of bodyguards grew from 300 members in 1925 to 50,000 in 1933 when Hitler took office. The man responsible for this growth was Heinrich Himmler, who commanded the SS from 1929 until its disintegration in 1945. Between 1934 and 1936, the SS gained control of Germany's police forces and expanded their responsibilities. Because of these new responsibilities, the SS divided into two sub-units: the Allgemeine-SS (General SS), and the Waffen-SS (Armed SS). Combined, these two organizations consisted of over 250,000 troops by 1939.
The General SS dealt with local police matters and with "racial matters." The main component of the General SS was the Reichssicherheitshauptamt ("RSHA," Reich Security Central Office in German). The RSHA itself was divided into four sub-groups, including the well-known Gestapo, headed by Heinrich Müller. The RSHA also dealt with foreign espionage and counterintelligence.
The Armed SS consisted of three main groups. The first was the Leibstandarte, Hitler's personal bodyguard. The second was the Totenkopfverbände (Death's-Head Battalions), which was in charge of the death and concentration camps. The third group was the Verfügungstruppen (Disposition Troops), an elite combat unit known for its extreme fighting tactics, which was mixed in with Germany's regular army. Because of the distasteful nature of their duties, members of the SS were schooled for many years in racial hatred, and were encouraged to harden their hearts to human suffering.
Unlike the SA ("Sturmabtelung," German for Assault Division, also known as "Brownshirts), who were considered to be a separate paramilitary organization working for the good of the State, the SS was under Hitler's total control. Easily recognizable by the lightning-shaped "S" insignia on their black uniforms, they soon became known as the purest of all Germans. To facilitate this, Himmler demanded that each officer prove the racial history of his family dating back to 1700. As the SS grew and became more complex, it matured into the spine of the Nazi regime.
Sources: Gutman, Israel. Encyclopedia of the Holocaust. "SS." Volume 3. NY: Simon and Schuster. 1990.