It's not a tax that will be shown on your ticket - but airfares will have to rise to cover the additional costs.
In effect the airlines serving Europe had to declare their actual CO2 output during 2010. In 2012 they will receive 'carbon-credits' that cover 85% of what they produced in 2010. If they exceed this 85% figure they will have to buy 'carbon-credits' on the free market to cover any excess.
The objective obviously is to encourage a reduction in overall CO2 output but it is overly complicated, based on questionable data (Eurocontrol guesses), and will benefit the Europeans at the cost of the rest of the world. (i.e. An airline buys a new fuel-efficient plane, where is it going to put it? Domestic routes, or routes to Europe). The Chinese are particularly angry as they are just beginning to catch of to the Europeans it terms of capacity, frequency, etc. It is going to cost them a great deal of money to grow, rising their costs to equal or exceed the European airlines costs.
It would have been much simpler to just add $0.10 tax on fuel purchased in Europe. While contrary to the Chicago Convention 1944, so is this scheme (despite the ruling of the ECJ which does not have the competency to rule on international law)