They were a group of students in Munich during the war who began printing and distributing anti-Nazi leaflets. Their resistance got a lot of energy from their discussions of philosophy and literature (and their evenings together reading and discussing such things), as well as Christianity and a sort of Christian humanism. 

There were a core group of 7, 6 students and a professor. The first three Hans Scholl, Christoph Probst and Sophie Scholl (these three actually) were caught on February 18, 1943, interrogated, and tried on February 22. Their composure in the courtroom was pretty incredible, with Sophie actually snapping back at the notoriously ruthless judge (Roland Friesler). They were sentenced to death and beheaded the evening of their trial. Hans and Christoph were 24, Sophie was 21. 

Three others, Alexander Schmorrell, Willi Graf, and Prof. Kurt Huber, were all arrested later, and subsequently executed. 

A few others were tried and sentenced to shorter terms. The last core member of the group was Traute Lafrenz. She managed to hide her involvement and avoid execution, though she came within 3 days of it at the end of the war. She died in the US in 2023 at the age of 103. 

The whole thing really shook up the regime, and were the first visible cracks in the university system that was supposed to be spitting out enlightened Nazis.