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From here: http://semanticstudios.com/publications/semantics/000029.php

Useful. As practitioners, we can’t be content to paint within the lines drawn by managers. We must have the courage and creativity to ask whether our products and systems are useful, and to apply our deep knowledge of craft and medium to define innovative solutions that are more useful.
Usable. Ease of use remains vital, and yet the interface-centered methods and perspectives of human-computer interaction do not address all dimensions of web design. In short, usability is necessary but not sufficient.
Desirable. Our quest for efficiency …