Characteristics of H-1B Specialty Occupation Workers: Fiscal Year 2015 Annual Report to Congress, October 1, 2014 - September 30, 2015 [open pdf - 7MB]
"The H-1B nonimmigrant classification is a vehicle through which a qualified alien may seek admission to the United States on a temporary basis to work in his or her field of expertise. An H-1B petition can be filed for an alien to perform services in a specialty occupation, services of an exceptional nature relating to a Department of Defense (DOD) cooperative research and development project or coproduction project, or services of distinguished merit and ability in the field of fashion modeling. [...] Determinations of whether to admit an alien in H-1B status are made by U.S. Customs and Border Protection at a port of entry. Possession of a visa does not guarantee admission."
U.S. Citizenship and Immigration Services: http://www.uscis.gov/