There is so much to living in the modern world that you have no control over. It's almost impossible for most people to live so green that they leave no footprint behind. People may need to drive cars, travel by plane, and even buses and subways have an impact. Some products you purchase may come in plastic containers, whether you want them to or not. It's unlikely you compost all your waste, and you have little control over what the garbage company or sewage treatment plant does. The list is endless.
To cover the balance of your footprint, you can purchase Green Credits, which offset the footprint you leave behind, despite your best efforts.