The term carbon footprint refers to the total amount of carbon dioxide (CO2) attributable to the actions of an individual (mainly through their energy use) over a period of one year. The term owes its origins to the idea that a footprint is what has been left behind as a result of the individual's activities.
The rapid build up of carbon dioxide in the atmosphere is now causing a cumulative warming effect on our planet. A carbon footprint is a representation of the effect an individual has on the earths climate.

For example, when you drive a car, each gallon of gasoline you burn produces carbon in the form of carbon dioxide. Depending on the fuel efficiency of your vehicle and the miles traveled, a gasoline-powered car can easily generate its own weight in carbon dioxide each year.

The average American is responsible for about 20 tons of carbon dioxide emissions each year, a far greater per capita number than that of any other industrialized country. In fact, the US accounts for more than 20% of the world's total greenhouse gas emissions.
Using public transit instead of driving alone is an easy way to reduce your personal carbon footprint.