The way programmers must build software is often a given, dictated by various technical and business constraints. The model for how the software actually works is called the implementation model. The way users perceive the jobs they need to do and how the program helps them do it is their mental model of interaction with the software. It is based on their own ideas of how they do their jobs and how computers might work. The way designers choose to represent the working of the program to the user is called the represented model, which, unlike the other two models, is an aspect of software over which designers have great control. One of the most important goals of the designer should be to make the represented model match the mental model of users as closely as possible. It is therefore critical that designers understand in detail the way their target users think about the work they do with the software.
[ref: About Face 2.0]
