Buick is a premium brand of General Motors

General Motors

General Motors Company , commonly known as GM, formerly incorporated as General Motors Corporation, is an American multinational automotive corporation headquartered in Detroit, Michigan and the world's second-largest automaker in 2010...

 (GM). Buick models are sold in the United States, Canada, Mexico, China, Taiwan, and Israel, with China being its largest market. Buick holds the distinction as the oldest active American make. Some current Buick models are shared with GM's German Opel

Opel

Adam Opel AG, generally shortened to Opel, is a German automobile company founded by Adam Opel in 1862. Opel has been building automobiles since 1899, and became an Aktiengesellschaft in 1929...

 subsidiary.
Buick is currently the oldest American still-active automotive make, and among the oldest automobile brands in the world.