What exactly is a carbon offset? In theory it's reduction in carbon emissions somewhere - through some action like planting a tree or paying for the spinning of a wind turbine - equal to your own emissions. Sounds easy, but the reality is very messy since the mechanisms for creating and buying emissions reductions and credits are incredibly varied. Here are four key questions to ask as you begin to think about your strategic choices in the (as-yet) mainly voluntary world of carbon offsets. By Andrew Winston