Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. The visa allows a foreign citizen, to travel to the United States port-of entry and request permission of the U.S. immigration inspector to enter the U.S.
There is a difference between a visa and a status, although both are referred to in the same manner and with the same alphabetical designation (based on the respective section of the Immigration and Nationality Act). A visa is simply a document in the person’s passport. It serves as a “ticket” to ensure that a foreign national can board the airplane to the U.S. A person’s visa status is both the category of admission and the time an individual may remain in the United States and is granted by the U.S. Department of Homeland Security once the applicant arrives at the border or a port of entry, and can be changed or extended by the USCIS at one of its remote Service Centers.