Models and Theories in Human-Computer Interaction/Affordances vs Conventions
Norman uses the term Concordance when speaking about intuitive design. His idea is that every detail of a product that is meant to be operated by a person should give a hint on how this product is supposed to be used. If it is a button - it is supposed to be pushed. If it is a handle - it is supposed to be grabbed and possibly turned. Ideally, the user should not be able to operate the product wrong way. For example, if doors open by pushing on them, there should not be any knobs or handles that user could grab and pull.
Very often we apply our memory and cognitive models when using objects. For example, not all screens are touch screens and none of them have an affordance to touch it. However, if the user has an experience with touch screens, she is trying to touch other screens she is dealing with. Now it becomes conventions, not affordance, because it is not obvious that the system can be operated by touching the screen. We know that only from our previous experience.
We use conventions in graphic design all the time. Blue underlined text on a web page does not give us a clue on how to use it. So, it is not an affordance. Only with experience we learn that if we see blue underlined text we can click on it and another page should open. When this is in the knowledge of the majority it becomes a convention.