In the animal kingdom, there are two major types that consume plants as a regular part of their diet: herbivores and omnivores. The major difference between the two is that while herbivores subsist on a diet made up exclusively of plants, omnivores consume a much more varied diet, and usually eat both plants and animals regularly. Omnivores are not to be confused with carnivores, which, like herbivores, survive on just one major type of food. Carnivores live primarily on a diet exclusively of meat.
When someone thinks of plant-eating animals, they likely think of an herbivore. They eat nothing but plants, and that can include grasses, trees, fruits, and even algae, a bacteria that obtains its energy via photosynthesis as plants do. Herbivores exist everywhere animals are present, and common types include cows, horses, giraffes and iguanas.
Living on a more varied diet, omnivores rely on plants as a steady means of energy, but also consume meat on a regular basis. While there have been cases of herbivores eating meat and carnivores eating plants in drastic circumstances, omnivores are classified as such because they regularly rely on both food types as part of a standard diet, as opposed to only in special circumstances. Many types of mammals are classified as omnivores, including raccoons, pigs, chimpanzees, and, yes, even humans. Several types of birds, like chickens and crows, are also classified as omnivores, as they eat both seeds and plant products, but also small bugs and grubs.
Carnivores have been known to eat plants when unable to track down a reliable supply of meat for their diet. However, they are not generally considered to be plant-eating animals. Any plant consumption by a carnivore occurs in dire circumstances when meat is not available, rather than a preferred dietary option. Common carnivores include cats and dogs.