In this paper, we first present a brief review of approaches used for studying and designing Augmented Reality (AR) systems. The variety of approaches and definitions in AR requires classification. We define two intrinsic characteristics of AR systems, task focus and nature of augmentation. Based on these two characteristics, we identify four classes of AR systems. In addition our OP-a-S notation provides a complementary characterization method based on interaction. Using OP-a-S, an AR system is modeled as a set of components that communicate with each other. One crucial type of OP-a-S component is the Adapter that establishes a bridge between the real world and the virtual world. By defining a classification scheme, we aim at providing a better understanding of the paradigm of AR and at laying the foundations of future design principles according to the class of systems. Keywords Classification. Interaction characterization.