Nowadays, visualization of large amounts of data has become a key issue for processing data in many fields. This paper presents a conceptual model for describing information visualization systems. This conceptual model has been represented using the Unified Modeling Language, therefore it takes advantage of the understandability, unambiguity, flexibility, and adaptability provided by this notation. The proposed model outlines concepts such as visualization workspace, device, visual representation and its features, information visualization task and its effects on visualization, inter-item relationship, etc. The main applications of the proposed model are: (1) to guide the development of a new visualization system by specifying which are the main concepts that have to be considered and how they relate to each other; (2) to characterize or describe an already existing visualization system; and (3) to thoroughly compare existing visualization systems. To illustrate the use and applications of the model, several examples of information visualization systems are provided.