There are primarily three categories of visas for individuals who seek to come to the United States : nonimmigrant visas, immigrant visas, and visas for refugees.

Individuals who come legally to the U.S. must be prescreened by a consular officer to obtain a "visa" that gives them permission to seek admission at the border. The visa, however, does not provide a guarantee that the person will be admitted. Furthermore, regardless of the category of visa, the length of time a person may remain in the U.S. will be governed by the permission given when admitted by a Department of Homeland Security (DHS) official upon inspection at the port of entry.

Whereas immigrants and refugees enter the United States with the intention of remaining permanently in the United Sates, most nonimmigrants come with a temporary purpose. Unlike the restrictions on the number of immigrants who may enter the United State during a specified year, for most nonimmigrant visas there are no numerical restrictions on the numbers of individuals who may be admitted, due to the temporary nature of the visit.

The Refugee Act of 1980 provides for relief from persecution: refugee protection, asylum, witholding of removal (known as nonrefoulement: a person may not be returned to a place where she faces persecution), protection from torture, and temporary protected status.