Carnivores need teeth that help them tear apart meat and bones, so they have sharp, scissorlike chompers. Herbivores, in contrast, don't need teeth that can tear but instead need grinding teeth, and so as a result they have many flat molars in the sides and back, while the front teeth make basic cuts to plants. Differences in teeth are just one way in which carnivores and herbivores differ.
A carnivore's entire mouth is designed to tear meat from bone so that the eater receives as much nutrition as possible from feeding. The jaws of a carnivore move up and down in an almost completely vertical motion, and its teeth are long, pointed and sharp. The fact that a carnivore's mouth is designed for quick ripping rather than grinding is mirrored by the contents of a carnivore's stomach, which uses enzymes about 10 times as acidic as that of a human's to digest food. The stronger acid is necessary because a carnivore's teeth do not nearly do as much grinding as an omnivore's.
An herbivore's teeth, in contrast, are designed to grind grasses, branches and seeds to prepare them for digestion. Plants don't try to escape, like small animals do, so the herbivore's teeth can focus more on making the bites small and easy to digest.