A visa is a document issued by the U.S. Department of State at an Embassy or Consulate abroad. Your visa does not guarantee that you will be admitted to the United States, but it does indicate that you have been investigated by the U.S. government and found to be eligible for the visa category stated on the face of the visa. The Customs and Border Protection agent will conduct an interview and make the final determination at the time of entry.
Application for Admission
Your visa does not give you permission to enter the United States it simply gives you permission to ask permission to enter. If you attempt to enter the United States without a visa you will most likely be detained and then placed on the next flight home if you are at an airport, or driven back across the border if you are at a border post.
Immigrant Visas are issued by U.S. embassies and consulates abroad. They permit the holder to apply for Permanent Admission to the U.S. at a border post, or more often, a port of entry - usually an airport. When entry is granted the visa holder becomes a lawful permanent resident and can live permanently in the United States. Evidence of Permanent Residence is in the form of a credit card size document often called a "green card" - they were not green for many years but they are green again.
Non Immigrant Visas
Non Immigrant Visas are issued by U.S. embassies and consulates abroad. They permit the holder to apply for Temporary Admission to the U.S. at a border post, or more often, a port of entry - usually an airport. Evidence of lawful admission is in the form of a stamp in the foreign passport and the issuance of a card I-94 which states the entry place, the entry date, the class of admission at the time of entry called "status" and the final date to leave the United States if nothing else changes.