Diets are a way to classify dinosaurs, based on food source.
Carnivores eat small lizards, other dinosaurs, and fish. All aquatic dinosaurs are carnivores.
Herbivores eat ferns and trees. Trees are only consumable in spring and summer.
Omnivores eat both meat and plants, meaning all food sources listed above are valid sources of food. However, most omnivores aren't able to reach tree leaves. Also, occasionally when an omnivore inspects a fern, it will provide the carnivore tooltip.