Ruminants have  a complicated digestive system in which the stomach is typically divided into four chambers. Some of the well known ruminants are cattle, sheep, goats, deer, antelope, etc.
The first chamber of the stomach is called the rumen. The second, third and last chambers are reticulum, omasum and abomasums respectively. The rumen is also called the pauch. It acts as fermentation chamber where food mixed with saliva undergoes fermentation by mutualistic or symbiotic microorganisms (such as bacteria, protozoa and fungi). These cellulose-digesting microbes live in the rumen of the  ruminants.
The rumen leads to the reticulum where the cud is then formed into palettes. The reticulum leads to the omasum. The abomasum which is the true stomach and corresponds to the stomach in humans. It leads to the duodenum which connects it to the small intestine.