Models and Theories in Human-Computer Interaction/Norman's Affordances and Use in Design
The dictionary definition of affordance is “the qualities or properties of an object that define its possible uses or make clear how it can or should be used - We sit or stand on a chair because those affordances are fairly obvious.” Don Norman talks about affordances many times in his book “The Psychology of Everyday Things” while giving great examples.
Norman also talks a bit about perceived affordance. In design (web, mobile, etc) we care much more about what the user perceives than what is actually true. As a designer, we care about if the user perceives that some action is possible or not possible. Some perceived affordances in design include using conventional usage in imagery and interactions. For example, a progress bar on a website. The perceived affordance is to move from left to right but what if someone reversed that? Then it becomes confusing and users would be unsure of what’s going on.
Another good example of this is using words to describe the desired action. If something is a link instead of just underlining it use words like "click here" with the link or using action words on buttons like “Add” or “Import” versus something like “Next."
In the end what matters is if the desired controls can be perceived in an easy to use design and if the actions can be discovered (and not hidden) and whether standard conventions are used. It is possible to break some of these rules, but majority of the time these affordances should be followed for a useful site.