A carbon footprint is "the total set of greenhouse gas (GHG) emissions caused by an organization, event or product." It is made up of the sum of the primary footprint and the secondary footprint.
The primary footprint is a measure of our direct emissions of CO2 - from the burning of fossil fuels (domestic energy consumption, transportation) and other energy consuming aspects of our life we have direct control over our primary footprint.
The secondary footprint is used to measure the the indirect CO2 emissions from the lives of the products we us - including their manufacturing and eventual breakdown. The more we buy, the more our secondary footprint is. We have limited control over our secondary footprint although we do have the ability to choose what we buy and how much of it we buy.
The new ecological 'fad' as such is to be carbon neutral. One can reduce their carbon footprint by using energy saving lightbulbs, public transport and any other form of reducing consumption. Furthermore one's carbon footprint may be reduced by countering one's carbon footpint with deeds that positively contribute to the environment such as raising awareness starting environmentally friendly products, introducing being environmentally friendly to large consumer companies and other pro-ecological acts. In order to reduce one's carbon footprint's one must engage in events that counter this increase and eventually engage in so many acts that counter our carbon emissions in order to achieve being carbon neutral (having a carbon footprint of 0).