“A user is a person who utilizes a computer or network service. Users of computer systems and software products generally lack the technical expertise required to fully understand how they work.”

A user often has a user account and is identified to the system by a username.

(wikipedia: user)

User-centered design (UCD) or user-driven development (UDD) is a framework of processes (not restricted to interfaces or technologies) in which usability goals, user characteristics, environment, tasks and workflow of a product, service or process 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 envision the way users are likely to consume a product, but also to validate their assumptions with regard to the user behavior in real world tests.

(wikipedia: user-centered design)

In user-centered design, archetypes and personas are created to represent the types of users. It is sometimes specified for each persona which types of user interfaces it is comfortable with (due to previous experience or the interface’s inherent simplicity), and what technical expertise and degree of knowledge it has in specific fields or disciplines. When few constraints are imposed on the end-user category, especially when designing programs for use by the general public, it is common practice to expect minimal technical expertise or previous training in end users.