I'm reading Apple's Human Interface Guidelines about Mental models and I can't understand what is the difference between Mental and Conceptual models.
I prefer a short answer if possible :)
"A mental model is the representation that a person has in his mind about the object he is interacting with. A conceptual model is the actual model that is given to the person through the design and interface of the actual product." (Susan M. Weinschenk. 2011. 100 Things Every Designer Needs to Know About People)
So a mental model is the way the person thinks about what it is they are doing. For example, when getting a book out of the library, they form a mental model of the things they have to do to achieve this.
A conceptual model for an online library is the interface the person interacts with as a represented concept of a library.
They are not the same (nor it is mandatory to be the same). The important thing is they are aligned (there are not inconsistencies between them).
For example, the accelerator pedal in a car is perceived by the designers as a means to inject more fuel to the car engine. However, the driver perceives it as a means to go faster (ignoring the mechanical implications). Both visions are different but consistent.
They are, effectively, the same thing in the context of technology which interfaces with users.
Apple is using the two terms rather loosely and interchangeably in the Interface Guidelines:
Don Norman explains the original Designer's Model / System Image / Users Model distinction next to the graphic about 'Conceptual Models' as part of this article below:
Don originally made the destinction about conceptual models (of how technology works) being just one type of mental model.