Carbon footprints?

Over the last few weeks I've been reading in blogs, and hearing people on the radio, talking about "carbon footprints".

I imagine a coal miner or someone working in a coal yard leaving big black footprints of coal dust on the pavement.

But then I hear about people boarding aeroplanes leaving carbon footprints. Surely the airline wouldn't let them dirty the carpets like that? Do they have diamond-studded shoes, or something.

Can anyone tell me what "carbon footprint" means?

I checked my dictionary, and it should have come between "carbonless paper" and "carbon monoxide", but didn't. But judging by the frequency with which I've heard it, it's becoming common currency, so it would be useful to know something about its meaning and origin.

I'm not averse to learning new words and phrases. After all, it was only last week that I heard "egregore" for the first time, and it seems to be quite a useful word, so maybe "carbon footprint" will be equally useful.