The Gestapo, abbreviated from Geheime Staatspolizei (German for Secret State Police), were the official secret police of Nazi Germany during WWII.
Formed in 1933, the Gestapo was created to uphold the rights of Nazi Germany's 'Aryan race' and kill anyone who got in the way of it, including Jews. On April 26th Hermann Goring was given command of this new force until Heinrich Himmler, already in charge of the SS, took command later that month.
A law enacted on February 10th 1936 made the Gestapo a 'Supreme Reich Agency'. giving the organisation absolute control over the internal security of Germany, Following the occupation of mainland Europe, this was extended to include the occupied territories.
At the 1946 Nuremberg Tribunal, the Gestapo was declared to be a criminal organisation, which meant that merely being a member was against the law.