What is a blanket made for? We lay them on our bed and sleep under them to keep us warm at night. We might also wrap up in a blanket and watch a movie or read a good book. Those typical ways of using a blanket would be known as perceived affordances. But if you were to ask my 3 year old what you do with a blanket he would have many different ideas. He likes to use a blanket for those same things as well but to him a blanket can be so much more than that. He can use it to hide under, drape over cushions to make a fort, lay on it to be swung in or pulled around on, or even tie over his shoulders to make a superman cape. All of those would be affordances. It is other ways that something can be used that weren't what it was originally intended to do.
When we are designing things we need to always take into account the difference between affordances and perceived affordances. If we just assume that the user will know how something is intended to be used then our design won't be as good as it could be.
"In design, we care much more about what the user perceives than what is actually true(Norman). "
One way to be sure that we are designing for the user with perceived affordances in mind is to test. Usability testing can offer designers so much information and provide them with the best insight to how well their design is doing. I really liked the quote from Steve Krug when he said, "The point of testing is not to prove or disprove something. It's to inform your judgment(Krug, 2006)." I think people get confused in thinking that testing will tell them if one option is better than the other option when in reality it is just to help you form a better decision about how to design. I am starting to realize that a little more as I do my own testing and study about it.
References:
Norman, Donald. "Affordances and Design."
RSS 20 Web. Jnd.org, n.d. Web. 02 Feb. 2013.
Krug, Steve. "Chapter 9."
Don't Make Me Think!: A Common Sense Approach to Web Usability. Berkeley, Calif: New Riders Pub., 2006. N. pag 135. Print.