To enter the United States, all nonimmigrant international visitors (except Canadians) are required to have the proper visa stamp placed in their passports. “Nonimmigrant” means a person has no intention of staying in the U.S. permanently. Visas are obtained at a U.S. Embassy or Consulate abroad. Visas cannot be obtained inside the U.S., since it is an "entry" document only.
People come to the U.S. for many different reasons, and the type of visa you request should match the purpose of your visit. Visa types are classified using an alpha-numeric system. For example, a visitor coming to study in the U.S. may be given an “F-1” or “J-1” student visa classification. A person coming to the U.S. for travel may be given a “B-2” visa, otherwise known as a tourist visa.
What is the purpose of a visa?
A visa allows you to travel to a U.S. port of entry (an airport, for example) and present yourself to a U.S. immigration inspector. The inspector will ask you some questions about your intentions for coming to the U.S. and check to make sure you have the appropriate visa. Once admitted, you will be given another document, called the I-94 Arrival/Departure Record, which indicates the specific nonimmigrant status you are allowed to use and the amount of time you are allowed to stay.
A J-1 Fulbright student should only enter the U.S. with the visa that has your current school's (or program) name noted on it, even if the visa that names your previous institution has not yet expired. If you change schools or programs, obtain a visa with the new program name noted on it before entering or re-entering the U.S.
Visa Expiration and Your Length of Stay in the U.S.
Although a visa has an expiration date, it does not determine how long you may remain in the U.S. (a visa is an ENTRY document only). Once you are in the U.S., there are other factors that determine your length of stay. International visitors coming to the U.S. as F-1 or J-1 students are generally allowed to remain for the length of their academic programs. However, if you exit the U.S. with an expired visa and need to re-enter, you will need to renew your visa while outside the U.S.