The rumen consists of a complex ecosystem where nutrients consumed by ruminants are digested by fermentation process, which is executed by diverse microorganisms such as bacteria, protozoa, fungi, bacteriophage and oscillospira. A symbiotic relationship is found among different groups of microorganisms due to the diverse nature of these microbial species and their adaptability and interactions. The ruminant provides necessary environment for the establishment of such microorganisms, while the microorganisms obtain energy from the host animal through microbial fermentation end products. The rumen microbial ecosystem fulfills several functions like fibrolytic, lipolytic and proteolytic functions and produce metabolites including volatile fatty acids (VFA), biohydrogenated lipids, microbial protein, methane etc. The purpose of this review is to contribute a better understanding of the fermentation processes that are taking place in the rumen and to provide information that can be applied for the development of new nutritional strategies to improve the digestion process for achieving maximum production.