up a computer for a school. You might want certain files to be accessible
only to teachers, not students, so you put all the teachers in a single
group. Then you can tell the system that certain files belong to members
of the group teachers, and that no one else can access those files.

Let's explore groups on the system. First, you can use the groups command
at the shell prompt. This will show you a list of the groups to which you
belong. Here's an example:

$ groups

system-wide configuration!permissions!file
ownershipusername dialout cdrom floppy audio

It's likely that you're a member of only one group, which is identical to
your username. However, root can add you to other groups. The above
example shows a person that is a member of five groups.

less /etc/group
This file lists the groups that exist on your system. Notice the root
group (the only member of this group is the root user), and the group that
corresponds to your username. There are also groups like dialout (users
who are allowed to dial out on the modem) and floppy (users who can use
the floppy drive). However, your system is probably not configured to make
use of these groups. It's likely that only root can use the floppy or the
modem right now. For details about this file, try reading man group.

ls -l /home
This command shows you that every user's directory is owned by that user
and that user's personal group.