Courtesy of Nat Torkington, via Hard News:
Offsets are easy to explain: you shit and it goes into your septic tank, but your backyard can't take it all the years of your turds. So you pay someone to truck your turds away and empty out the tank. You burn fuel (releasing greenhouse gases) or make things that rot (releasing greenhouse gases) or have some other chemical process that releases greenhouse gases like CO2. The Earth can't take it all so you pay someone to take your carbon away and empty out the atmosphere. The only way to do this that we've got at the moment is to plant trees. (If someone develops a "sequestration" system, aka burying the carbon back in the earth and thus out of the atmosphere, it'll qualify as an offset too)Emissions trading is the same, except that you also have the option of paying other people not to shit. At which stage, the scatological metaphor breaks down...Polluting = shitting. Atmosphere = your back yard septic tank. Offset = paying someone to truck it away.
