What is the SA (Brownshirts) as noted in the biography, Adolf Hitler?

The SA, or Brownshirts is the main street army of the Nazis during the period of their organization and coming to power. After Hitler comes to power, the army is afraid that the Brownshirts will takeover the army. During the Night of the Long Knives, SA leader Ernst Rohm is executed without a trial. Later, the SA has only a minor role in military education.