Green energy refers to energy that has been generated without the burning of fossil fuels. It can also include nuclear derived energy.
Your energy provider will confirm what their energy mix is and whether they provide a green tariff but do check the small print to see whether you are happy with the product. A more accurate term used by most energy providers is 'renewable' energy.
Many energy providers now guarantee that they source all of their electricity from renewable sources, such as wind, hydro and solar.