The concepts and terms commonly used to talk about systems have not themselves been organized into a system. An attempt to do so is made here. System and the most important types of system are defined so that differences and similarities are made explicit. Particular attention is given to that type of system of most interest to management scientists: organizations. The relationship between a system and its parts is considered and a proposition is put forward that all systems are either variety-increasing or variety-decreasing relative to the behavior of its parts.