A teamster can refer to a few different things:

• In American English, a teamster is a truck driver or a person who drives teams of draft animals, such as oxen, horses, or mules, pulling a wagon.
• The International Brotherhood of Teamsters is a labor union in the United States and Canada that represents a diverse membership of blue-collar and professional workers in both the public and private sectors, totaling about 1.3 million in 2015. The union was formed in 1903 by the merger of the Team Drivers International Union and the Teamsters National Union.
• The Teamsters are known as the champion of freight drivers and warehouse workers, but have organized workers in virtually every occupation imaginable, both professional and non-professional, private sector and public sector. They are Americas largest, most diverse union, with nearly 1,900 affiliates throughout the United States, Canada, and Puerto Rico.

In summary, a teamster can be a truck driver, a person who drives teams of draft animals, or a member of the International Brotherhood of Teamsters, a labor union that represents a diverse membership of workers in various sectors.