from Wikepedia...

A carbon footprint is a "measure of the impact human activities have on the environment in terms of the amount of green house gases produced, measured in units of carbon dioxide"^[1] It is meant to be useful for individuals and organizations to conceptualize their personal (or organizational) impact in contributing to global warming. A conceptual tool in response to carbon footprints are carbon offsets, or the mitigation of carbon emissions through the development of alternative projects such as solar or wind energy or reforestation. A carbon footprint can be seen as a subset of earlier uses of the concept of ecological footprints.

To calculate your carbon footprint....

http://www.carbonfootprint.com/calculator.html