Users, User IDs And Effective User IDs.

When does a user come into existence? - this happens as soon as an entry for this user is added into the '/etc/passwd' file.

A process running in the system carries along a user ID. normally, this is the ID of the user that started this process.

Exceptions to the above are daemon processes that run as 'root', and later switch to the ID of a different user. Only 'root' processes may normally perform the switch en-route...