A visa is the authorization by the U.S. government to allow a foreign national to enter the United States for a specific duration and for a specific purpose. For example, a B-2 visa would allow someone to enter the United States for a two-week vacation, whereas an H-1B visa would allow someone to work for a specific employer for up to three years. In contrast, a green card is an approval by the U.S. government to allow a foreign national to permanently live and reside in the United States. This would permit him/her to live, work or study freely without any restrictions.