The original use of the term Budweiser was used to refer to beer from the city of Ceske Budejovice in Bohemia (the modern Czech Republic) since 1265. Derived from the German name for the town, Budweis, something from Budweis being Budweiser.
The largest (but not the only) brewery in Ceske Budejovice has long exported beer under the "Budweiser" brandname, and in many European countries is the only beer that can be sold as "Budweiser". In some places where it competes with the American Budweiser it is marketed by the brewer Budejovicky Budvar using the names "Budvar", "Budweiser Budvar", and "Budejovicky Budvar". It recently started having limited distribution in the USA under the name "Czechvar".
The United States Budweiser is the best-selling brand of beer sold by Anheuser-Busch, the world's largest brewing company, based in Saint Louis, Missouri. As the name implies, it was originally brewed in imitation of the famous Czech beer, but over the decades has developed its own style which has proved very commercially successful. It is brewed in the lager style using mass production techniques. Budweiser beer deteriorates quickly with age or excessive heat, so to ensure quality the company has fairly stringent requirements for handling and sale. There are numerous Budweiser breweries located around the nation, many of which offer tours with free samples.
Anheuser-Busch has a market share in the United States of about 50%, for all of its brands of beer combined. The company is known for its sports sponsorships, video game sponsorship (Tapper), and (often) humorous advertisements. Advertising campaigns have included frogs saying 'bud-wei-ser', lizards doing the same, Clydesdale horses, etc. It is also known for its repeated legal attempts to obtain complete control of the Budweiser trademark, which have been generally unsuccessful. The inexpensive and nearly colorless and flavorless beer itself takes a backseat to the advertising.
In the United States and most of Latin America, the American beer is the only one which can be sold under the name "Budweiser". In some countries where the Czech brew has rights to that name, the American beer is sold as "Bud".