This is the nerve center of your system, it contains all system related configuration files in here or in its sub-directories. A "configuration file" is defined as a local file used to control the operation of a program; it must be static and cannot be an executable binary. For this reason, it's a good idea to backup this directory regularly. It will definitely save you a lot of re-configuration later if you re-install or lose your current installation. Normally, no binaries should be or are located here.
/etc/X11
This directory tree contains all the configuration files for the X Window System. Users should note that many of the files located in this directory are actually symbolic links to the /usr/X11R6 directory tree. Thus, the presence of these files in these locations can not be certain.
/etc/adduser.conf
'adduser' configuration. The adduser command can create new users, groups and add existing users to existing groups. Adding users with adduser is much easier than adding them by hand. Adduser will choose appropriate UID and GID values, create a home directory, copy skeletal user configuration from /etc/skel, allow you to set an initial password and the GECOS field. Optionally a custom script can be executed after this commands. See adduser(8) and adduser.conf(5) for full documentation.
/etc/apt
This is Debian's next generation front-end for the dpkg package manager. It provides the apt-get utility and APT dselect method that provides a simpler, safer way to install and upgrade packages. APT features complete installation ordering, multiple source capability and several other unique features, see the Users Guide in /usr/share/doc/apt/guide.text.gz
/etc/apt/sources.list
Contains a list of apt-sources from which packages may be installed via APT.
/etc/bash.bashrc
System wide functions and aliases' file for interactive bash shell.
/etc/crontab
'cron' configuration file. This file is for the cron table to setup the automatic running of system routines. A cron table can also be established for individual users.
/etc/devfs
This daemon sets up the /dev filesystem for use. It creates required symbolic links in /dev and also creates (if so configured, as is the default) symbolic links to the "old" names for devices.
/etc/fstab
The configuration file for 'mount' and now 'supermount'. It lists the filesystems mounted automatically at startup by the mount -a command (in /etc/rc or equivalent startup file). Under Linux, also contains information about swap areas used automatically by swapon -a.
/etc/group
It lists the configured user groups and who belongs to them.
/etc/gshadow
Contains encrypted forms of group passwords.
/etc/hostname
Contains the hostname of your machine (can be fully qualified or not).
/etc/hosts
This file is used to define a system name and domain combination with a specific IP address. This file needs to always contain an entry fr an IP address, if the machine is connected to the network.
/etc/modules
List of modules to be loaded at startup.
/etc/mtab
List of currently mounted filesystems. Initially set up by the bootup scripts, and updated automatically by the mount command. Used when a list of mounted filesystems is needed, e.g., by the df command. This file is sometimes a symbolic link to /proc/mounts.
/etc/postfix
Holds your postfix configuration files. Postfix is now the MTA of choice among Linux distributions. It is sendmail-compatible, offers improved speed over sendmail, ease of administration and security. It was originally developed by IBM and was called the IBM Secure Mailer and is used in many large commercial networks. It is now the de-facto standard.
/etc/passwd
This is the 'old' password file, It is kept for compatibility and contains the user database, with fields giving the username, real name, home directory, encrypted password, and other information about each user. The format is documented in the passwd man(ual) page.
/etc/rc?.d
These directories contain all the files necessary to control system services and configure runlevels. A skeleton file is provided in /etc/init.d/skeleton
/etc/rcS.d
The scripts in this directory are executed once when booting the system, even when booting directly into single user mode. The files are all symbolic links, the real files are located in /etc/init.d/. For a more general discussion of this technique, see /etc/init.d/README.
/etc/sudoers
Sudoers file. This file must be edited with the 'visudo' command as root. The sudo command allows an authenticated user to execute an authorized command as root. Both the effective UID and GID are set to 0 (you are basically root). It determines which users are authorized and which commands they are authorized to use. Configuration of this command is via this file.
/etc/shadow
Shadow password file on systems with shadow password software installed (PAMs). Shadow passwords move the encrypted password from /etc/passwd into /etc/shadow; the latter is not readable by anyone except root. This makes it more difficult to crack passwords.
/etc/security
Essential to security. This subdirectory allows administrators to impose quota limits, access limits and also to configure PAM environments.
/etc/shells
Lists trusted shells. The chsh command allows users to change their login shell only to shells listed in this file. ftpd, the server process that provides FTP services for a machine, will check that the user's shell is listed in /etc/shells and will not let people log in unless the shell is listed there. There are also some display managers that will passively or actively (dependent upon on distribution and display manager being used) refuse a user access to the system unless their shell is one of those listed here.
/etc/skel
This directory contains configuration files and subdirectories for the setup of system configuration specifics and for the boot process, like 'clock', which sets the timezone, or 'keyboard' which controls the keyboard map. The contents may vary drastically depending on which distribution and what utilities you have installed. For example, on a Redhat or Mandrake based system it is possible to alter an endless array of attributes from the default desktop to whether DMA should be enabled for your IDE devices. On our Debian reference system though this folder is almost expedient containing only two files hwconf and soundcard which are both configured by the Redhat utilities hwconf and sndconfig respectively.