I like to picture it this way: The National Grid is like a great big reservoir of electricity, like a reservoir of water. There are lots of different electricity suppliers pumping electricity into the reservoir, and lots of users (like you and me and businesses and industries) taking electricity out.
All electricity is the same so it doesn't matter if it all gets muddled up.
You can choose whether you are a customer of "Dirty Old Coal-Fired Electricity PLC" or whether you are a customer of "Nice Clean Wind Energy Inc". But either way you take electricity out of the big reservoir (the national grid). The difference is that your chosen supplier will measure how much electricity you took out, and they will put the same amount back in, either from a coal-fired power station or from a wind farm (or nuclear power station, hydroelectricity farm, or a number of different options). Then they bill you for what you used.
This is an oversimplification but I don't think it's a bad one. I'm looking forward to being corrected by your comments, though!
Just to make things a bit more complicated there are a few different green energy schemes you can sign up to. Some use your money to buy 100% renewable energy from existing schemes. Some will use your money to invest in new renewable energy schemes. Some don't give renewable energy at all, but make a donation to the RSPB on your behalf. But don't worry, if you go to uswitch.com they explain it all very clearly.
Please visit the uswitch website and spend a few minutes investigating the options for changing your electricity supplier. This is really easy folks - even easier than changing to low-energy light bulbs or not leaving your TV on standby or all the other challenges we've ever had. I'm asking for five minutes of your time to slash your carbon emissions forever. Don't forget to vote in the poll in the right-hand sidebar wen you've done it.