Wikipedia explains it best:
United Services Automobile Association (USAA) is a Fortune 500 financial services company focused on providing banking, investing, and insurance to people that have served in the U.S. military (including the Coast Guard), selected federal agencies, and their families. It was founded in 1922 by a group of U.S. Army officers to self-insure each other when they were unable to secure auto insurance due to the perception that they were a high-risk group. It has since expanded to serve officers, enlisted, and their families a full range of property & casualty insurance, banking, life insurance, and investment services. USAA was one of the pioneers of direct marketing and most of its business is conducted over the Internet or telephone.