A labor union is an organization of workers joined to protect their common interests and improve their working conditions. It serves as an intermediary between the employer and the employees. The main purpose is to give workers power to negotiate more favorable working conditions through collective bargaining. Some of the largest and/or most prominent unions in the U.S. include the United Auto Workers, Service Employees International Union, International Brotherhood of Teamsters, American Federation of State, County and Municipal Employees, and the United Steelworkers. The AFL-CIO (member list) and Change to Win (member list) are federations, large umbrella organizations of unions which have banded together to share resources and promote common political and organizing goals.