Molecular gastronomy is a scientific discipline involving the study of physical and chemical processes that occur in cooking. It pertains to the mechanisms behind the transformation of ingredients in cooking and the social, artistic and technical components of culinary and gastronomic phenomena in general (from a scientific point of view).
There are many branches of food science, all of which study different aspects of food such as safety, microbiology, preservation, chemistry, engineering, physics and the like. Though until the advent of molecular gastronomy, there was no formal scientific discipline dedicated to studying the processes in regular cooking as done in the home or in a restaurant. The aforementioned (perhaps with the exception of food safety) have mostly been concerned with industrial food production and while the disciplines may overlap with each other to varying degrees, they are considered separate areas of investigation.
Though many disparate examples of the scientific investigation of cooking exist throughout history, the creation of the discipline of molecular gastronomy was intended to bring together what had previously been fragmented and isolated investigation into the chemical and physical processes of cooking into an organized discipline within food science to address what the other disciplines within food science either do not cover, or cover in a manner intended for scientists rather than cooks.