Here’s a good overview of usability I found on a Facebook group - User Centred Design (UCD).

http://www.medicalcomputing.org/archives/0agui.php

From the article:
‘Whether you aspire to develop the next big software hit or simply create computer applications for your personal and office use, your applications will need effective user interfaces to fulfill their potential. Designing such an interface is part discipline (following platform conventions and good design principles), part science (usability testing) and part art (creating screen layouts that are informative, intuitive and visually pleasing).
…
The answer: functionality and interface. If functionality is what a program actually does, then interface is how the user interacts with the program, perceives how the program does its work. Concealing the details of a complex technology behind a good task-oriented interface is not a new concept. One does not have to understand the inner workings of an automobile to drive one.’

If only it were that easy!