A user is a person who uses a computer or network service. Users generally use a system or a software product without the technical expertise required to fully understand it. Power users use advanced features of programs, though they are not necessarily capable of computer programming and system administration.
A user often has a user account and is identified to the system by a username (or username). Other terms for username include login name, screen name, nickname or handle.
Some software products provide services to other systems and have no direct end users.