Human intestinal microbiota has undergone a lot of research in recent years and we are beginning to expand our knowledge of the role of bacteria, fungi, viruses and helminths in our gut in health and disease. Our gut contains myriad of over hundreds of millions of microbes that influence our body physiology, nutrition, metabolism, and immunity. A change in the type of these microbes has been shown to be associated with gastrointestinal diseases such as inflammatory bowel disease and systemic diseases like metabolic syndrome and cancer. In this article, an attempt has been made to review some significant recent studies that have looked into the complex types of intestinal microbes and how they interact with the human body. This paper also serves to explain what probiotics are, and how they function as a separate "organ" within our body. It includes their method of action, the latest evidence-based research on their use in health and disease and some common sources of probiotic bacteria.