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 1003.1-1996]
A "group name" is a string that is used to identify a set of users. A "group id" is a non negative integer, 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, a group ID value is referred to as a real group ID, or an effective group ID. [POSIX 1003.1-1996]