I’m not the biggest fan of his site, but this article is great — it explains mental models in Fisher Price terms.

From Jakob Nielsen:

A mental model is  what the user believes about the system at hand.

Note the two important elements of this definition:

• A mental model is based on belief, not facts: that is, it’s a model of what users know (or think they know) about a system such as your website. Hopefully, users’ thinking is closely related to reality because they  base their predictions about the system on their mental models and thus plan their future  actions based on how that model predicts the appropriate course. It’s a prime goal for designers to make the user interface communicate the system’s basic nature well enough that users form reasonably accurate (and thus useful) mental models.
• Individual users each have their own mental model. A mental model is internal to each user’s brain, and different users might construct different mental models of the same user interface. Further, one of usability’s big dilemmas is the common gap between designers’ and users’ mental models. Because designers know too much, they form wonderful mental models of their own creations, leading them to believe that each feature is easy to understand. Users’ mental models of the UI are likely to be somewhat more deficient, making them more likely to make mistakes and find the design much more difficult to use.

No matter what you think, your users have a different idea what the site does than you do.

And…

Remember Jakob’s Law of the Internet User Experience: Users spend most of their time on websites  other than yours. Thus a big part of customers’ mental models of your site will be influenced by information gleaned from other sites.

A huge duh moment. What do you mean they spend more time at places other than my restaurant?