Report about Usability, in which it is explained this concept. The report describes and analyzes different aspects on applications design and user-centered services. The document is divided in three big chapters. In the first one it is analyzed what usability is and it’s described the relationship of this term with other similar ones, like experience of the user and information architecture. The second chapter studies the human factor and it explains how the users use an application or service, their visual perception, cognition, mental models and social nature. The third chapter is about the User Centered Design (UDC) and are described the main research techniques, evaluation and documentation. It includes an extensive bibliography.