The Order of St. Augustine was one of the great mendicant orders of the Middle Ages (the others were the Dominicans, the Franciscans, and the Carmelites). The order began officially in 1243, when Pope Innocent IV issued a papal bull uniting several hermit communities that professed to follow the Rule of St. Augustine.
The Order's character mainly comes from the writings of St. Augustine, and their participation in the Mendicant movement. Since then, the Augustinians have placed their focus on missionary work, and working with the poor. The first Catholic missionaries in the Philippines were Augustinians, led by the explorer Andres de Urdaneta.
Perhaps the two most famous Augustinians are Martin Luther, who was a German Augustinian, and Gregor Mendel.
The official home page of the Order is http://www.aug.org/