The Psychopathology of Everyday Things
Affordance
Affordance is a concept proposed by American perception psychologist James J. Gibson — a noun-form of 'afford,' meaning 'to bestow' or 'to bring about.' It means that the impression of a situation or an object can naturally invite a specific action. A solid, gently sloped, wide surface, for example, naturally invites people to stand and walk on it. Donald A. Norman extended Gibson's concept through the lens of usability and applied it to design. Norman treats an object's perceived properties and real characteristics as affordances, and sees those as what determines how the object gets used. In industrial design and interaction design, affordance can also mean connecting different concepts.
Constraint
In affordance, a constraint is a limit so natural the user doesn't notice it — providing a cue for action. Types include physical constraints that limit what's operationally possible, semantic constraints based on situation and knowledge, cultural constraints that lean on cultural conventions, and logical constraints that depend on the corresponding action.
Mapping
In affordance, mapping refers to the relationship between the controls that exist in a design and the behavioral outcomes that follow. If the results and effects of an action land close to expectations, the mapping is good; if they deviate, the mapping is bad. When action and outcome are poorly mapped, the user feels a cognitive load.
Source: http://blog.daum.net/kinosine/16
