Primary consumers eat plants, we also call them herbivores. Some herbivores graze on plants and others browse. Carnivores are secondary consumers, they eat other animals. A thrush eating a snail is an example of a carnivore, as is a lion eating a zebra. Some animals can choose between being a herbivore and being a carnivore, they eat both animals and plants and we call them omnivores, they are either primary or secondary consumers.
Scavengers are also consumers, but they wait for their prey to die first.
A food chain is a very simple diagram to show how energy flows through an ecosystem. This food chain shows energy from the sun is used by grass, the cow gets its energy by eating the grass, and then we get energy by eating the cow or drinking its milk. A food web diagram lets you predict what will happen to one animal population if another population gets bigger or smaller. Providing the prey species reproduces as fast as it is predated, its population will stay at a constant size. If the rabbits in a grassland ecosystem reproduce faster than the foxes predate them, the rabbit population will increase.