Green cards and visas are both legal documents that grant foreign citizens residency in the United States. However, the biggest differences between the two are when they are acquired and how long they grant residency. A visa is acquired before coming to the U.S. It gives an individual the right to stay in the country, but only for a certain period. A green card, alternately, grants an individual lawful permanent residency. It is acquired once they already live in the country.