If you were to weigh all the mammals in the world, the sheer bulk mass of them would be herbivores. Plant material is mostly cellulose with water and some carbohydrates, but very little fats and proteins. Cellulose, however, is largely indigestible to vertebrates, particularly mammals as they do not have the correct enzymes to break it down. Mammals get around this problem by living in harmony with a variety of microbes. This internal flora breaks sown the cellulose into usable sugars and also secretes various vitamins and fatty acids.
Different mammals have their digestive tracts organised in different ways.
Thus one group of mammals known as ruminants has evolved several stomachs which are called the Rumen, Reticulum, Omassum and Abomassum. The first, known as the Rumen, is a huge reservoir in which chewed vegetation is attacked by the animal's internal flora. No digestive juices are secreted in this stomach. Cows are a prime example of this form of digestion. Many herbivorous mammals however, to not have a rumen - horses and kangaroos for instance.