The Design of Everyday Things by Donald Norman
Discover-ability: Is about the process of discovering an object/thing and working out its properties.
Understanding: After discovering something you need to understand how it works and how the phenomena can be understood.
The Six Big Things;
Affordances: Affordances are not things, or a thing in an object. It is something that appears when an object comes into a relationship with a person. So you could roughly say it is what the user gets out of it. For example a bottle has the possibility to contain water but it also has the affordance of letting you hold the bottle. An Anti Affordance is something you can’t do with an object.
Signifiers: Signifiers are things that helps us in a certain way. It is something that points at an affordance (see above), while signifies can communicate where the action should take place, so one cannot really exist without the other. It tells you what you can and can’t do with an object. For example it could be like on a packet of microwaveable rice and it says “Tear here” to show where you tear it to open it.
Mapping: It is to do with the position and placement of signifiers, and in the way that the controls relate to the things they control. For example the dials to control hotplates on a hob. Good mapping is the most effective way to show that the controls relate to the thing they are controlling, so related controls should be grouped together etc.
Feedback: It tells you if what you have done is correct or incorrect and does not need to be positive. For instance trial and error is valid, if somewhat unsatisfying, feedback. An example could be the tactile feedback on keyboards, especially mechanical keyboards, and letting you know if you’ve tapped the key with enough force for the keyboard to pick it up. Also on Windows if you do something the system that it can’t do you get a “ping” noise alert and some sort of pop up window. Too much feedback can be overwhelming, which in video-games is a common complaint leveled at some of the Assassin’s Creed games where the UI is cluttered and you begin to have no idea what half of it means.
Constraints: Constraints are things that prevent you from doing things. Constraints can be physical, semantic, cultural or logical. For example Nuclear Weapons might require two people to be physically able to turn a key and press a button to ensure no single person could do it by themselves. Also at a concert there will be security to protect the celebrity from crazy people. Something being nonsensical would be a semantic constraint while doing something that was highly illogical would be an logical constraint. For instance “I put the milk on the cereal then go to the supermarket and buy the milk” would be a logical constraint.
Conceptual Models: The mental image of how something works, a simplified and easy to explain version of something. It can be useful but you must be careful as it can be quite easy to confuse the prospective user so it is best to have broad strokes conceptual models that get the “Big ideas” across.