An immigration quota system is a way of limiting immigration based on the immigrant's country of origin; in the United States, it was initially brought into place by The Quota Act of 1921. This act placed ceilings on the amount of people allowed to immigrate from specific countries.
The immigration quota system enacted by the 1921 act has evolved into a much more complex system that incorporates a per country limit with preferential groups. Relatives of people living in the United States and people employed in specific industries are two groups that receive preferential treatment.
The United States still has a limit for how many people can immigrate from any given country, with exceptions often granted for individuals seeking asylum and refugees.