Perhaps you have observed cattle standing in a pasture doing nothing but chewing. They are not grazing, they are just chewing. This is called chewing their cud, but what exactly does that mean, and what kinds of animals chew their cud?
Animals who chew there cud are called ruminants, while cattle are the best known ruminants there are around 150 animals who exhibit the same traits for chewing their cud, including sheep, goats, bison, camels, llamas, deer, and antelope. Horses and pigs are not ruminants.
While it is often said that these animals have four stomachs, a more honest explanation is that the stomach of ruminant animals has four compartments. The compartments are the rumen, reticulum, omasum, and the abomasum. The rumen being the largest of these compartments.
Most ruminants do not have upper front teeth, they graze by pressing their lower front teeth on the hard pad at the top of their mouth. When a ruminant animal eats it simply is putting the food into its stomach with very little early chewing. The food is then mixed with saliva in the rumen and reticulum. The food then forms into solid lumps, called cud, which the animal can regurgitate later for chewing.
The cud is chewed slowly, with a side to side grinding motion, using their hind teeth, before being swallowed. Grass and plants are hard to digest so this second chewing is required to break the food down further for digestion. After food is swallowed the second time it can be passed into the other parts of the stomach, the omasum, and abomasum, the last of which is considered to be a true stomach.
As grass and plants offer very little digestibility to to animals, ruminants use another tactic to get their nutrition. There guts contain billions of yeast organisms, fungi, and protozoa, and even more bacteria. It is these organisms which are vital in the role of digestion of the plant cellulose, and when these organisms die the ruminant animal is actually able to digest them.
The whole digestion process produces a lot of gas, methane in particular, and ruminant animals are often said to be especially flatulent. Sheep belches and cattle belches are a major contributing factor to greenhouse gases.