Herbivores are forms of heterotrophs (things that must eat something else to survive because they do not undergo photosynthesis or chemosynthesis) that only eat plants. This is easy to remember since the word "herb" is in the word "herbivore" and herbs are plants! In visual representations that show energy flow within an ecosystem (such as food chains and food webs), herbivores are sometimes referred to a primary consumers.
This hyperlinked picture shows a food web that would be characteristic within Lake Michigan. The herbivores would be the organisms that have only green arrows pointing towards them. They include the native water flea, calanoids, rotifers, molluscs, and zebra (or quagga) mussels. Small fish, duck, moose, and deer would also be included as herbivores that are characteristic to the areas near Lake Michigan.