Normally, the IRS collects income taxes from everyone living and working in the U.S., regardless of citizenship. A few lucky foreigners, however, are allowed to live and work in the U.S. without paying a dime in income tax.
Individuals working in the U.S. on behalf of a recognized international organization fall into this category. Check out this list of organizations designated as tax-exempt by the executive order of various American presidents. Among them are well-known entities like the World Health Organization and the United Nations. But there's also the Pacific Salmon Commission and the International Fertilizer Development Center.
Employees of these international organizations, plus their immediate families, are exempt from U.S. income taxes. The same exemption goes for foreign citizens who hold diplomatic visas to work in consulates and embassies. Both the diplomats and their families (spouses and children under 21) are exempt from U.S. income taxes.