Here is a brief explanation of affordance.
“The word “affordance” was originally invented by the perceptual psychologist J. J. Gibson (1977, 1979) to refer to the actionable properties between the world and an actor (a person or animal). To Gibson, affordances are a relationship. They are a part of nature: they do not have to be visible, known, or desirable. Some affordances are yet to be discovered. Some are dangerous. I suspect that none of us know all the affordances of even everyday objects.” -Don Norman, http://www.jnd.org/dn.mss/affordances_and.html
Affordance is all possible uses for a thing. These can be visible and unseen. Such as an affordance of TV programs is that people will think about or learn from them but that is all internal unless the subject makes it known. Knowing the afforances of things is more important to the designers than users. The thing being designed need to be able to afford or allow the intended use. There are near infinite possible affordances as such perception of what is affordanble is more important and controllable.
This argued by Don Norman is more important than all actual possible uses. Think of it, if the designed object is not perceived at all to have the uses it doe,s chances are few will figure out it purpose such as a new invention very unlike anything seen before. Also if the perceived affordance for this object has strong cultural constraints than they may not realize its purpose is for something completely different such as taking a common object and having it look like 1 thing but be not at all meant for it such, as a toaster that is really an alarm clock and has no heating element but pops up and dings when the alarm is done.
“In similar vein, because I can click anytime I want, it is wrong to argue whether a graphical object on the screen “affords clicking.” It does. The real question is about the perceived affordance: Does the user perceive that clicking on that location is a meaningful, useful action to perform?” -Don Norman ,http://www.jnd.org/dn.mss/affordances_and.html
Perceived Affordances of a screen
A screen affords many things including licking and clicking but the designer needs to make sure the perceived affordances is not to lick but to click and to click the correct spots at times with tool as a mouse. This is done with such things as visual cues showing you there is something on the screen. If there is something people may try to interact but that isn’t enough. Feed back also helps by having the mouse change to a pointy hand when over a web link so know you can interact with it also there is at the bottom of web browsers when you hover a link at the bottom the actual web address appears. Cultural constraints help in that a link maybe underlined, have a http on it or being the standard blue color when not clicked. There are many good ways to make a thing work and have perceived affordances but there are more ways to break a design and make it hard to understand.
As graphic designers we deal with perceived affordances mostly in interaction design. Designers do need to be sure the perceived afforances match actual affordances and that those two are not used in such a way so to make catastrophic failure. Such as in don normans, design of everyday things, he mentions a glass wall at a buss stop. Glass has the perceived use of shattering so it and was shattered but when replaced with wood it was not broken thought had a similar affordance to being broken with the force. Woods perceived as having strength and not shattering while glass is perceived as fragile and shatter able.