Visa
A visa is a stamp that is placed in a passport by a U.S. consular official. Visas serve only one purpose — they allow an individual to travel to the U.S. border to request (re)admission into the United States on a particular immigration status. Visas have no bearing on the length or validity of a person's stay in the United States. However, a valid visa is required to reenter the United States after international travel, with a few exceptions.
Citizens of Canada and the British Overseas Territory of Bermuda are exempt from visa requirements; however, they still hold immigration status and are subject to the same laws that govern their immigration status.
Immigration Status
An individual’s immigration status is what controls their ability to stay in the United States and the types of activities they are permitted to engage in while in the United States. Immigration status is principally controlled by the I-94. As of May 2013, U.S. Customs and Border Protection (CBP) is no longer issuing these cards at the port of entry. Rather, you receive a stamp in your passport indicating the status on which you’ve been admitted and for how long you can stay in the United States. You can then access your I-94 information on the U.S. Customs and Border Protection website.
Duration of Status (D/S)
Individuals who are admitted in J-1 status will have I-94 information that reads J-1 D/S (Duration of Status). This means that a J-1 is legally present in the United States provided they have a valid passport, a valid DS-2019 form, and continue to engage in activities that are consistent with their immigration status.
DS-2019 Visa Document
The visa document is what establishes the who, what, where, when, and how of immigration. It lists the biographical data of the individual, the activities they will be undertaking, the location of that activity, the dates of that activity, and the immigration sponsor.