The 'Red Orchestra' was an umbrella term adopted by the Nazi secret police when describing those who were plotting against the Third Reich. The term 'Red Orchestra' was used because each person in the plot was given a musical term - so that some were known as pianists, leaders were known as conductors. The 'Red Orchestra' was not a unified front but rather a collection of different resistance movements united by the sole desire to rid Nazi Germany of Adolf Hitler. The 'Red' part of the tile came from the movement's links to communism and the USSR. It is probable that there were no more than 100 people in the units involved.
The 'Red Orchestra' was made up of three different units: the Trepper unit, the 'Red Three' and the Schulze-Boysen/Harnack group. The Trepper unit was based in Germany, France and Belgium , the 'Red Three' was based in Switzerland while the Schulze-Boysen/Harnack group was based in Berlin.
German security forces were so concerned about the 'Red Orchestra' that they on this rare occasion grouped their resources so that the SD, the Gestapo and the Abwehr worked together.
The purpose of the Trepper unit was to gather intelligence about the Nazi's military power in Western Europe. Led by Leopold Trepper, the units - there were seven parts to it - proved very successful at infiltrating the Nazis and found out about troop deployment and even new tank designs. Some Trepper agents were given passes that allowed them free movement through Nazi occupied Western Europe. Trepper was arrested on December 5th 1942 but managed to survive the war- which most of his comrades did not.
The Schulze-Boysen/Harnack group was formed by a Luftwaffe officer, Harro Schulze-Boysen. He had opposed Hitler for a long time but joined the Luftwaffe to give himself some sort of cover. He joined the Nazi Party in 1937 to further increase his cover. Numerous people from all sections of society joined the group - including Horst Heilmann who was a cipher officer for OberKommando.
With such a large and eclectic group it was only a matter of time before German intelligence broke into it. Before the Schulze-Boysen/Harnack group was broken up it gave intelligence findings to the US embassy in Berlin. It also posted anti-Nazi posters in railway stations and helped people flee Germany. Schulze-Boysen himself was arrested on August 30th 1942 and the movement broke up.
Red Three was outside of the jurisdiction of the German intelligence service as it was based in Switzerland. It was headed by a communist called Alexander Rado and provided the Allies with much useful intelligence material. After the war, Rado was recalled to the USSR and imprisoned after being found guilty of spying for the UK. He was released on the death of Stalin.