"This Office of Immigration Statistics Annual Flow Report examines the number and characteristics of nonimmigrant admissions in 2005 recorded by the Treasury Enforcement Communications Systems (TECS) of the Department of Homeland Security's (DHS) U.S. Customs and Border Protection (CBP). Each year, millions of foreign nationals are admitted into the United States. Those who are admitted temporarily for a defined period of time and for a specific purpose are referred to as nonimmigrants. Most nonimmigrants enter as tourists or business travelers on short-term trips, while others, such as students, temporary workers, and foreign diplomats, stay for longer periods of time, ranging from a few months to several years."
US Customs and Immigration Services: http://www.uscis.gov/