Because of OS X’s Unix heritage, it has a multi-user permission model on the file system.
Every user on the system has an integer user ID. Each user belongs to one or more named groups, and each group has an ID. For instance:
ls -l chapter.txt-rw-r--r-- 1 markd staff 48827 Aug 11 14:14 chapter.txt
The user is markd, the group is staff.
ls -l /bin/ls-r-xr-xr-x 1 root wheel 80688 Feb 11 2010 /bin/ls*
ls is owned by the user root, the group wheel.
You can see what users and groups are configured on your machine with the Directory Services tools dscl and ...