A "user name" is a string that is used to identify a user. A "login name" is a user name that is associated with a system login. A "user id" is a non negative integer, which can be contained in an object of type uid_t, that is used to identify a system user. When the identity of a user is associated with a process, a user ID value is referred to as a real user ID, or an effective user ID. [POSIX 1.003.1-1996]
A "group name" is a string that is used to identify a set of users. A "group id" is a non negative interger, which can be contained in a object of type gid_t, that is used to identify a group of system users. Each system user is a member of at least one group. When the identity of a group is associated with a process, agroup ID value is referred to as a real group ID, or an effective group ID. [POSIX 1003.1-1996]
The format of the User and Group databases is not specified. Programs may only read these databases using the provided API. Changes to these databases should be made using the provided commands.