Green Tariffs: how green are they?

I have been looking at the websites of various green energy suppliers, but it is in many cases actually quite hard to work out what the energy mix is and just what they mean when they say that your energy is a given-percentage "green".

What do they mean by green and how does it work - are they actually giving you green energy there and then or are they simply going to use your money to build green generators, or are they doing something confusing like trading certificates?

I wondered if there is anywhere to find out this sort of more detailed information in order to make an informed choice about the various tariffs.