According to Wikapedia:

"A carbon tax is a tax on energy sources which emit carbon dioxide into the atmosphere. It is an example of a pollution tax, which some economists favor because they tax a "bad" rather than a "good" (such as income). Because a carbon tax addresses a negative externality, it is classed as a Pigovian tax, named after Arthur Pigou, who first proposed targeted taxation as a corrective to externalities.

Because of the link with global warming, a carbon tax is sometimes assumed to require an internationally administered scheme; that is not intrinsic to the principle, however. The European Union has discussed a carbon tax covering its member states to supplement the carbon emissions trading scheme begun in January 2005. However, emissions trading systems do not constitute a Pigovian tax insofar as (a) the payment for emissions is not received by a governmental body, and (b) the price per unit of emissions is not fixed as it is in tax systems, rather it is a market price that fluctuates.

The purpose of a carbon tax is environmental, to reduce emissions of carbon dioxide and thereby slow global warming. It can be implemented by taxing the burning of fossil fuels — coal, petroleum products such as gasoline and aviation fuel, and natural gas — in proportion to their carbon content. Unlike market-based approaches such as carbon cap-and-trade systems, it has the benefit of being easily understood and can be popular with the public if the tax is hypothecated to fund environmental projects[1]. "

Martin