Dumbledore's Army is a group consisting of Gryffindors, Ravenclaws and Hufflepuffs. They were created by Harry Potter and his friends in an effort to learn defense against dark magic when the Ministry of Magic wouldn't allow students to be taught the subject. After the members were caught, Harry was nearly expelled until Dumbledore took responsibility for the organization. The group was disbanded after that.
They were initially in opposition of Dolores Umbridge. During the Second Wizarding War, it was in opposition of Voldemort's rule.