File ownership

Each file is 'owned' by a user and a group:

 $ touch somefile
 $ ls -l somefile; ls -ln somefile
 -rw-rw-rw-  1 reegen seattle      0 Apr 19  2000 somefile
 -rw-rw-rw-  1 1001   500          0 Apr 19  2000 somefile
                 |     |
                 |     \- group ownership
                 |
                 \- user ownership

 $ id
 uid=1001(reegen) gid=500(seattle) groups=500(seattle)

The kernel doesn't ever see symbolic names, just the uid/gid.
User to uid mapping is in /etc/passwd.
Group to gid mapping is in /etc/group.