'Usability' is refered mainly with respect to user interface of information system, for example, design of Web site. It is defined in ISO 9241 (http://www.iso.org/) as follows :
"Usability is a measure of the effectiveness, efficiency and satisfaction with which specified users can achieve specified goals in a particular environment."
In fact, 'usabiltiy' concerns to all of information type, infomation design, user interface design and diversity of individuals.
The point is that human factors must be considered from both sides of common features and diversity.
For example, "hesitant users (contrasted to frequent users)" is a problem to be approached in this way. - Distinction between can't and won't becomes essential.
The inquiry of usability takes course of (1) reducing personnel requirements and (2) reducing training requirements.
Remark : Users considered in the subject of 'usability' include contents creators. As to contents creators, the design guideline becomes issue in such a way as if they will not follow guidelines, chaotic collections of documents result which cannot be accessed/navigated.