The term "carbon footprint" is used to describe the amount of greenhouse gases (GHG) that are emitted into the atmosphere each year by an entity such as a person, household, building, organization, or company. It is usually measured in units of carbon dioxide equivalents.
A carbon footprint usually includes both direct and indirect GHG emissions:
Direct emissions are considered to be emissions that are directly under the control of the person or company. For example, when a person drives his or her car it emits GHGs. For a company, if it chooses to heat its factory by burning a fuel, those are direct emissions.
Indirect emissions are emissions that are a consequence of the activities of the reporting entity, but that occur at sources owned or controlled by another entity. For example, purchased electricity is considered an indirect emission. While a person or company can control the amount of electricity that is purchased, they cannot control the emissions that are associated with the generation of that electricity. Those emissions are under the direct control of someone else.
For example, a person with a small carbon footprint would engage in activities that limit the amount of GHGs he or she emits in the course of a year. Carbon dioxide is the primary GHG emitted in industrialized countries, and emissions of other GHGs are often expressed in terms of their equivalent value in carbon or carbon dioxide. A person's carbon footprint would include all the GHG emissions associated with his or her activities, not just those of carbon dioxide.
We are not sure of the origin of the term, although it is probably an extension of the concept of "ecological footprints" developed by the ecologist William Rees at the University of British Columbia in the early 1990s. Dr. Mathis Wackernagel, executive director of the Global Footprint Network, states that the phrase "carbon footprint" received "its biggest boost in 2005 through an enormous BP media campaign on the carbon footprint." Taken from New York Times, February 17, 2008, William Safire: On Language: Footprint. http://www.nytimes.com/2008/02/17/magazine/17wwln-safire-t.html
EPA's climate change website is a good resource for more information on climate change and greenhouse gas emissions: http://www.epa.gov/climatechange/