A nonimmigrant is a foreign citizen present in the United States (U.S.) on a temporary basis. A non-immigrant visa is required for entry into the U.S. and authorizes a stay for a limited period of time for a specific purpose. Once a person is admitted as a nonimmigrant, they are restricted to the activity for which they were admitted. The type of visa required is defined by immigration law, and relates to the principal purpose of the travel. Foreigners come to the U.S. for a wide variety of reasons, including tourism, business, study and transit.
Bureau of Consular Affairs, travel.state.gov/visa/temp/temp_1305.html
Becoming a Nonimmigrant Student in the United States, U.S. Immigration and Customs Enforcement, www.ice.gov/sevis/becoming_nonimmigrant_student_52007.htm#_Toc129683762