"U.S. law provides for the temporary admission of various categories of foreign nationals, who are known as nonimmigrants. Nonimmigrants are admitted for a designated period of time and a specific purpose. They include a wide range of visitors, including tourists, foreign students, diplomats, and temporary workers. There are 24 major nonimmigrant visa categories, and 72 specific types of nonimmigrant visas issued currently. These visa categories are commonly referred to by the letter and numeral that denotes their subsection in the Immigration and Nationality Act (INA), e.g, B-2 tourists, E-2 treaty investors, F-1 foreign students, H-1B temporary professional workers, J-1 cultural exchange participants, or S-4 terrorist informants. Interest in nonimmigrant visas soared immediately following the September 11, 2001 terrorist attacks, which were conducted by foreign nationals apparently admitted to the United States on legal visas. Since that time, policy makers have raised a series of questions about aliens in the United States and the extent that the federal government monitors their admission and presence in this country. Some of the specific visa categories are the focus of legislative activity (e.g., guest workers)."
CRS Report for Congress, RL31381
United states Department of State: http://fpc.state.gov/