Usable web sites are logical, intuitive, and clear to the people who use them.
This is an important distinction because programmers and designers (people who typically develop Web sites) statistically have richer than average spatial conceptualization skills. More simply put, it's easier for designers and programmers to understand and "keep track" of where they are in an application's "space" without additional navigational cues -- so a Web site that seems easy to navigate to a designer or programmer may be confusing or unclear for a "regular" person