What is the difference between adjustment of status and immigrant visa?

Under the USCIS guidelines, an immigrant visa is a document issued by a U.S. Consulate in a foreign country that allows someone to travel to the United States with an "immigrant visa" and get their green card once they arrive. Adjustment of Status, is the process in which an eligible individual ALREADY PHYSICALLY LIVING in the United States can apply to get a green card, in the U.S., WITHOUT having to return to their home country. CITATION; http://ciudadjuarez.usconsulate.gov/whatisaniv.html http://www.uscis.gov/green-card/green-card-processes-and-procedures/adjustment-status