What is a carbon footprint?

From the diagram and quote I think that Carbon footprint is a unit to measure the effects on the environment caused by human activities. Carbon footprint uses tonnes or kg of CO2 as unit (e.g. London Underground tubes have 10 tonnes of carbon footprint, which is not true)

“A carbon footprint is a measure of the impact our activities have on the environment, and in particular climate change. It relates to the amount of greenhouse gases produced in our day-to-day lives through burning fossil fuels for electricity, heating and transportation etc.
The carbon footprint is a measurement of all greenhouse gases we individually produce and has units of tonnes (or kg) of carbon dioxide equivalent”

The diagram and quote is taken from http://www.carbonfootprint.com/carbonfootprint.html