To be allowed into the U.S., all nonimmigrant international visitors (except Canadians) are required to have the proper visa stamp placed in their passports. "Nonimmigrant" means there is no intention of staying in the U.S. permanently. Visas are obtained at a U.S. embassy or consulate abroad. Visas cannot be obtained within 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 for 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. The sample here shows what a tourist visa looks like:
The visa allows you to travel to a U.S. port-of-entry (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, your arrival information will be entered into a database from which you can print your arrival record https://i94.cbp.dhs.gov/I94/#/home, called the I-94 Arrival/Departure record, which indicates which nonimmigrant status you are allowed to use and the amount of time you are allowed to stay.
A J-1 Exchange Visitor should only enter the U.S. with the visa that has the school's (or program) name noted on it (see annotation, above), even if the visa 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.
Although a visa has an expiration date, it does not determine how long you can 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.