Artificial Intelligence is a much wider term and includes many aspects of bringing intelligence to the machines with ultimate goal of making machines as intelligent as humans. This typically includes studies from many disciplines (computer science, philosophy, psychology, neurology, sociology etc) and interdisciplinary studies.Machine learning can be considered as a sub-branch of artificial intelligence which deals with designing statistical/mathematical models for decision making which can learn from the surrounding experience.Multi Agent systems typically focus on problems involving systems with multiple elements and the interactions among them. You should also look at Game Theory and related topics for better understanding of Multi Agent systems.