Usability Theory encompasses a group of ideas about how humans interact with technology. As one computing dictionary defines it, usability is “the effectiveness, efficiency, and satisfaction with which users can achieve tasks in a particular environment of a product. High usability means a system is: easy to learn and remember; efficient, visually pleasing and fun to use; and quick to recover from errors."
While the standards in that definition are more concrete, Usability Theory concerns the human factors that would either improve or hinder the ability to achieve the previously-mentioned standards. In other words, Usability Theory studies how humans learn and remember systems, how the human eye processes certain images and other aspects of the user-technology experience, and determines which type of system would allow them to do so the most efficiently and with the least struggle.