An user name (also called login name or logon) is a unique name for each user of computer services which can be accessed by several persons.
Users need to identify themselves for accounting, security, logging, and resource management. Usually a person must also enter a password in order to access a service. Once the user has logged on the operating system will often use a (short) user identifier, e.g. an integer, to refer to them rather than their user name.
User names can usually be any short string of alphanumeric characters. Common choices are first name, initials, or some combination of first name, last name, initials and an arbitrary number. User names are often assigned by system administrators according to some local policy, or they may be chosen by the users themselves.
Computer security is usually tied to user magamenent: a particular user (called Administrator or root) is given complete access to the system, while normal users have limitations on what they can do. Sometimes "users" are created for particular computer programs that need to access important features in their normal functioning or, in the opposite case, for programs that are likely to cause problems and must have as restricted access as possible to the system (the latter is the case for web servers, that are often by malicious users as a way to access the system).
User names are often also used as mailbox names in electronic mail addresses.