All living things need to constant find a source of energy to keep on going. How they get this energy into their system is referred to as nutrition. Some organisms only eat plants, like cattle. They are called herbivores. Some only eat meat like sharks. They are called Carnivores. You also have humans that do both and we are referred to as Omnivores. You also have other organisms that live off of dead or decaying materials called decomposers.
We spend a good bit of time here focused on Fungi. Let's face it fungi and decomposer nutrition, in general, is very interesting.