Any organism that eats only plant material. Herbivores are considered primary consumers in the food web and eat plants that absorb and store energy from the sun through photosynthesis. Carnivores, which are considered secondary or tertiary consumers prey upon herbivores.
A herbivore is often defined as any organism that eats only plantsDefinitions of herbivore on the Web http://www.google.com/search?hl=en&lr=&defl=en&q=define:herbivore&sa=X&oi=glossary_definition&ct=title. By that definition, many fungi, some bacteria, many animals, about 1% of flowering plants and some protists can be considered herbivores. Many people restrict the term herbivore to animals.