Interactivity has become ubiquitous in the digital media landscape. Numerous interactive tools are designed, tested, deployed and evaluated. Yet, we do not have generalizable knowledge about the larger concept of interactivity and its psychological impact on user experience. As a first step toward a theory of interface interactivity, this paper identifies three species of interactivity corresponding to three central elements of communication - source, medium, and message. Interactivity situated in any of these three loci of communication can provide cues and affordances that operate either individually or together to capture users' attention and determine the nature and depth of their processing of online content as well as contribute to their perceptions, attitudes and behavioral intentions. This paper discusses psychological mechanisms by which the three classes of interactivity tools affect users, with the specific purpose of drawing out design implications and outlining UI challenges for strategic development of interactive interfaces.