Creating user interfaces that meet the user's needs and expectations requires careful consideration of the actual context of usage. Especially in complex software or web designs it is hard to predict all user requirements and make specifications without thoroughly analysing the context of usage and visualising possible design solutions.
In broad terms, user-centered design (UCD) is a design philosophy and a process in which the needs, wants, and limitations of the end user of an interface or document are given extensive attention at each stage of the design process. User-centered design can be characterized as a multi-stage problem solving process that not only requires designers to analyze and foresee how users are likely to use an interface, but to test the validity of their assumptions with regards to user behaviour in real world tests with actual users. Such testing is necessary as it is often very difficult for the designers of an interface to understand intuitively what a first-time user of their design experiences, and what each user's learning curve may look like.
The chief difference from other interface design philosophies is that user-centered design tries to optimize the user interface around how people can, want, or need to work, rather than forcing the users to change how they work to accommodate the system or function.
UCD Models and Approaches
Models of a user centered design process help software designers to fulfill the goal of a product engineered for their users. In these models, user requirements are considered right from the beginning and included into the whole product cycle. Their major characteristics are the active participation of real users, as well as an iteration of design solutions.
All these approaches follow the ISO standard Human-centered design processes for interactive systems (ISO 13407 Model, 1999).
User-centered design according to Donald Norman
The book "The Design of Everyday Things", originally called "The Psychology of Everyday Things" was first published in 1986. In this book, Donald A. Norman describes the psychology behind what he deems 'good' and 'bad' design through examples and offers principles of 'good' design. He exalts the importance of design in our everyday lives, and the consequences of errors caused by bad design. However, his approach has been criticised for being overly functionalist by proponents of a critical design methodology where the experience of the user is considered beyond mere utility.
In his book, Norman uses the term "user-centered design" to describe design based on the needs of the user, leaving aside what he considers to be secondary issues like aesthetics. User-centered design involves simplifying the structure of tasks, making things visible, getting the mapping right, exploiting the powers of constraint, and designing for error. Norman's overly reductive approach in this text was redressed by him later in his own publication "Emotional Design". Other books in a similar vein include "Designing Pleasurable Products" by Patrick Jordan, in which the author suggests that different forms of pleasure should be included in a user-centered approachas in addition to traditional definitions of usability.
User-centered design focuses on more than just computers and single users
While user-centered design is often viewed as being focused on the development of computer and paper interfaces, the field has a much wider application. The design philosophy has been applied to a diverse range of user interactions, from car dashboards to service processes (such as the end-to-end experience of visiting a restaurant, including interactions such as being seated, choosing a meal, ordering food, paying the bill etc).
When user-centered design is applied to more than single user interactions, it is often referred to as user experience. A user experience comprises a number of separate interfaces, human-to-human contacts, transactions and conceptual architectures. The restaurant example (above) is an example of this - ordering a meal or paying the bill are two user interactions, but they are a part of the "user experience" called dining out. It is not enough to have the separate interactions that comprise an experience being usable. The goal is that each interaction should integrate with every other interaction that forms a part of a single experience. In this way, the experience as a whole is rendered usable.