DS-2019 - A form given to exchange visitors in the U.S. on J-1 visas. This form is the document which shows the start and end date of your visit, program of study, finances and is issued by the school you are required to attend.
I-20 - A form given to individuals in the U.S. on F-1 student visas. This form is the document which shows the start and end date of your visit, program of study, finances and is issued by the school you are required to attend.
I-94 - This document states your admissions number into the United States and indicates your immigration status when you enter the U.S. by the Customs and Border Patrol. Recently, the process changed from a paper copy stamped into your passport to an electronic format. The new format requires you to print out your I-94 document which can be done at the U.S. Customs and Border Protection web site.
Visa - Your visa is the stamp in your passport that indicates the status under which you are permitted to enter the country. It is used ONLY for entry into the U.S. F or J visas cannot be renewed or extended within the US. An F or J visa does NOT indicate the length of time you are allowed to remain in the United States, as the length of stay is indicated on your I-94 document.