UI Concepts

A user interface is well designed when the program behaves exactly how the user thought it would.

A user interface is well designed when the program model conforms to the user model.

A new user who sits down to use a program does not come with a completely blank slate. They have some expectations of how they think the program is going to work. This is called the user model: it is their mental understanding of what the program will do for them.

The program, too, has a model, only this one is encoded in bits and will be faithfully executed by the CPU. This is called the program model, and it is The Law. Nothing short of electrical storms and cosmic rays can convince a CPU to disobey the program model.

Let's look at another example. In Microsoft Word (and most word processors), when you put a picture in your document, the picture is actually embedded in the same file as the document itself. You can create the picture, drag it into the document, then delete the original picture file, but the picture will still remain in the document.
Now, HTML doesn't let you do this. HTML documents must store their pictures in a separate file. If you take a user who is used to word processors and doesn't know anything about HTML, then sit them down in front of a nice WYSIWYG HTML editor like Microsoft FrontPage, they will almost certainly think that the picture is going to be stored in the file. 

Call this user model inertia, if you will. So, we have an unhappy conflict of user model (the picture will be embedded) versus program model (the picture must be in a separate file), and the UI is bound to cause problems.