Usability is not one thing. It's actually a relationship between a program and its audiences. A program's usability changes as the audience changes; in particular, best practices for usability engineering change as its audiences' degrees of investment in the task and the software changes. We'll find it useful, later on, to distinguish at least four different kinds of user from each other.

The nontechnical end-user has very little knowledge of either the application domain of your program or of computers and software in general, and is probably not very interested in acquiring more of either. Most users of ATMs (automatic teller machines), for example, fall in this category.

The domain expert has a lot of domain knowledge about the application your program addresses, but is unsophisticated about computers. Domain experts are usually motivated to learn more about their domain, but not necessarily about computers in general.

The power user has a lot of experience with computers and different kinds of software, but limited domain-specific knowledge. Power users (like domain experts) are often willing to learn more about what they are already good at, but may resist acquiring more domain knowledge.

The wizard or guru is strong in both domain-specific and general computer knowledge, and is willing to work at getting more of either.

Of course, the degree to which any one persion is a domain expert varies with the domain. And even wizards don't want to have to be wizards all the time; even a software guru who enjoys programming (say) printer drivers usually just wants to press a button and have the printer print without fuss or fiddling.

Nevertheless, these four categories do define a power curve — nontechnical end-users at the low end, wizards at the high end, domain experts and power users in the middle. Their requirements and the kinds of interfaces that make them comfortable will differ. Just as importantly, an individual's requirements will change as he or she climbs the power curve.

In [TAOUP] we noted that while nontechnical end-users value ease most, wizards will trade that away to get expressiveness and concision. In this book we'll explain and analyze that tradeoff further, focusing on the tension between task-oriented and feature-oriented interfaces.