The /etc directory contains the all-important passwd file, which gives
important information about which users are configured on the system, what their
user ID numbers are, what their default groups are, where their home directories
are, and even what shells they use by default.
The default group is actually a group ID number rather than a name. However,
you can easily match the group ID to the group name by looking at /etc/group.
This also gives you a list of users broken down into the groups to which they
belong. Note that "groups" is plural because a user can belong to more than one
A list of the essential system files can be found here.