An energy crop is a plant whose sole purpose is to provide energy.
A crop of corn planted to provide food, for example, is not an energy crop if its waste residue is also recovered and used to generate energy. Biomass plants use waste to create renewable energy and are very important in rural German areas.
To stick with the example of corn, an energy crop used to produce biogas is actually harvested before the ears become ripe enough to eat, and the entire plant is used in the process. In contrast, only the fruit – the edible part – is used to make ethanol.