When a percieved characteristic of an object determines its action. For instance, the round characteristic of a wheel makes for better rolling. Square or triangular wheels do not roll as well. Therefore, round wheels afford
better rolling. In interaction design, affordance is widely used. Consider the arrow that you move around with your mouse to interact within the virtual space on your computer. The fact that it's an arrow shape, a well-known directional element, affords a better understanding of where you want to go on the screen. Another symbol in its place, such as a heart, has a different social meaning that does not afford the interaction. Many everyday objects afford, or demonstrate affordance. Some examples could be a pop can, a television remote, a car steering wheel, or a bar stool. (Lidwell, William et al. Universal Principles of Design. Rockport Publishers, Massachusetts. 2003)

The visual language necessary for distinguishing the difference between interface elements

An affordance is a visual clue that helps a user understand that something is an active interface element and what its function might be. For example, a button's beveled edge and its shadows are affordances, notifying a user that the shape is meant to be clicked by reminding them of keys on a keyboard, or buttons in an elevator. Such affordances tend to fall into two categories: those that mimic real life that usually involve 3-D modeling, and those that build upon a visual language unique to computers.
Obvious visual clues (like size), help to reinforce an element's usability. While size is not literally an affordance, the shape and proportion of a button can affect the perceived importance of an element. Very small buttons—smaller the 15 pixel square—are often missed entirely, while extremely large ones—larger than 100 pixel square—start to look like static elements.

Eaton, Eric. DesignWhys: Designing Web Site Interface Elements (Rockport Publishers, Inc., Massachusetts, 2003)