The GNU C Library
29.1 User and Group IDs
Each user account on a computer system is identified by a user
name (or login name) and user ID. Normally, each user name
has a unique user ID, but it is possible for several login names to have
the same user ID. The user names and corresponding user IDs are stored
in a data base which you can access as described in 29.13 User Database.
Users are classified in groups. Each user name belongs to one
default group and may also belong to any number of
supplementary groups. Users who are members of the same group can
share resources (such as files) that are not accessible to users who are
not a member of that group. Each group has a group name and
group ID. See section 29.14 Group Database, for how to find information
about a group ID or group name.