One particularly confusing concept in immigration law is the difference between "Visa" and "Status". These terms are often used interchangeably but they mean very different things.
The Visa is a physical document (or sticker) pasted to one of passport pages. This document allows a person to travel to a port of entry and seek admission to the United States under the category designated in the visa. Upon passing inspection at the port of entry, a visa holder is conferred their status in the United States. The Status represents the person’s rights and obligations during their time in the country. For example, a person holding an F-1 Student Visa can seek entry to the United States to attend school. Upon being admitted to the country, he or she would be conferred the F-1 Student status as their legal basis for being in the United States. That person must only engage in activities allowed to those under that status.
The first step to entering the United States is to qualify for a legal basis to do so. In other words, one must satisfy the requirements for a particular status. This can be done through filing a petition with the U.S. Citizenship and Immigration Service or through presenting documents to a U.S. embassy or consulate. When it is determined that the requirements for a particular status has been met, a visa can be issued on the basis of that status. With that visa, one can seek entry to the United States under that status. The status is conferred upon admission into the country.
It is also important to note the difference between a Visa’s validity and the validity of a Status. Since the duration of a person’s status is assigned at the time of admission, a visa may expire before the end of that status. Because the visa is an entry document, its expiration has no effect on the visa-holder so long as he or she maintains lawful status (doing what they are allowed to do) in the United States. The visa, however, would need to be renewed prior to the next time they enter the country. These circumstances are quite common because certain statuses—such as the F-1 Student—do not have a specific expiration date; they last as long as the qualified activities continue in the United States. (These types of status expiration dates are represented as "D/S", or "Duration of Status".)