What is the difference between an immigrant and a nonimmigrant visa (between a Green Card and a US Visa)?

An immigrant visa (Green Card) grants the privilege of living and working permanently in the United States. A nonimmigrant visa (US Visa) is issued to persons with permanent residence outside the U.S. but who wish to be in the U.S. on a temporary basis, for example, tourism, medical treatment, business, temporary work, or study.