In ecological psychology, the concept of affordance refers to the perception of opportunities for action in the surrounding objects. Problematically, the lack of conceptual unity pervades the characterization of the phenomenon, leading to important differences between the definition that different authors use of the concept. This paper offers a systematic exegesis of the original work of J.J. Gibson focused on the question about (a) what affordances are, and (b) what are their main components. Finally, it is offered a general classification that might guide the categorization of current uses of the term is proposed depending on the way in which different approaches conceptualize the relationship between these main components.