Normally, a carbon footprint is expressed in tonnes of carbon dioxide equivalent.
For a product, the carbon footprint includes all possible steps in the development, production, transport and disposal that give rise to carbon emissions. Both direct and indirect emissions are counted in the footprint. An example of a direct emission is the greenhouse gas released when something is produced. An example of an indirect emission could be the release of greenhouse gases during transportation of the product to its final destination.
Human activities that leave behind a carbon footprint include travel by any means using fossil fuel and using electricity from non-renewable sources. In fact, even breathing releases CO2 into the atmosphere. When the carbon footprint for an individual is calculated the time span is usually one year.
Lowering of the carbon footprint is the principal way to mitigate anthropogenic global warming. Since it is impossible to have no carbon footprint some people and companies try to offset their footprint by supporting reduction in greenhouse gas emissions elsewhere or by contributing to projects that can work as carbon sinks.