Agency

An agency is an organization which acts on behalf of others. There are commercial agencies, and government agencies. Dictionary.com defines 'agency': "an organization, company, or bureau that provides some service for another: a welfare agency". Actors, models, authors, sportspeople all have agents or agencies.

Bureau

A bureau is agency within an agency. For example:

• The United States Department of the Interior is responsible for preserving the land and forests, and state parks. An agency within the Interior Department, the Bureau of Indian Affairs has the job of providing certain services to native American and native Alaskan tribes.
• The United States Department of Homeland Security, tasked with protecting the country from foreign threats, uses the Federal Bureau of Investigation as a national police force against terrorism and treason.

United States

In the United States, government agencies include the United States Department of Homeland Security, which has the job of protecting U.S. citizens; the United States Department of Education, which sets certain rules for how U.S. schools are to be run; and the United States Department of the Treasury, which helps keep track of the nation's money supply.