Ruminants are a group of mammals special in the sense that they can get nutrition from cellulose rich plant based food.
They are herbivores and thus their daily diet is rich in plant parts which contain abundant cellulose which is indigestible by mammals.
The basic difference between ruminants and other mammals lies in the structure of stomach. Ruminant's stomach is very large and consists of four chambers namely rumen, abomasum, reticulum, omasum. Among these rumen is the largest chamber. They quickly ingest the food available without chewing it, and then regurgitate it when resting and chew it then. This process is called rumination. Symbiotic bacteria in their gut ferment the cellulose and the digestion product is absorbed by the ruminant mammal.