Here is a list of important configuration files and directories in Linux, I hope it will help you as quick reference.

/etc/paswd – Contains a list of the systems accounts, username and passwords.

/etc/group – This file contains basic group attributes
/etc/shadow – This is file with encrypted password & its parameters like expiry, last change etc.
/etc/profile – The user’s initialization file
/etc/inittab – Contains all information regarding runlevels and system initialization
/etc/fstab – The fstab file typically lists all available disks and disk partitions and mount the specified device.
/etc/skel – Contains all the default system initialisation files
/etc/grub.conf – GRUB: GRand Unified Bootloader configuration file
/boot/grub/menu.lst – This is linked to /boot/grub/grub.conf
/etc/securetty – This file allows you to specify which TTY devices the root user is allowed to login on.
/etc/sysconfig/network – This file is used to specify information about the desired network configuration
/etc/sysconfig/network-scripts/ifcfg-ethN (files) – Configuration files for each network device
/etc/host.conf – Resolver library to obtain the IP address corresponding to a host name.
/etc/hosts.allow – Allow access to TCP-wrapped Services with this file.
/etc/hosts.deny – Deny access to TCP-wrapped Services with this file.
/etc/resolv.conf – This file Contains details of the DNS server
/etc/hosts – Contains a list of IP addresses and the hostnames that they correspond to.
/etc/sysconfig/iptables – IPTABLES firewall configuration file
/etc/sysconfig/selinux – SELinux configuration file.
/etc/init.d – Contains all the daemon files
/etc/init – This is a file linked to /sbin/init file (used to change run levels)
/etc/xinetd.d – Directory contains the configuration files for each service managed by xinetd and the names of the files correlate to the service
/etc/rc.d/rc.sysinit – The first processes that init starts specified in this script.
/etc/rc.d/rc.local – Local system boot processes can be placed in this file, useful for system customization.
/etc/rc0.d – Contains scripts for run level 0
/etc/rc1.d – Contains scripts for run level 1
/etc/rc2.d – Contains scripts for run level 2
/etc/rc3.d – Contains scripts for run level 3
/etc/x11/prefdm – Xwindow display-manager configuration file.
/etc/x11/xorg.conf – This file used for configuring the X.Org Server.
/etc/x11/xinit/xinitrc – X session script.
/etc/sysctl.conf – This file contains kernel options related to networking security.
/etc/cups – Directory contains printer configuration files
/etc/bootrc – For changing timeout of booting process. For example set timeout = 10
/etc/auto_master – File responsible for auto mounting
/etc/auto_home – Contains file-names responsible for auto mounting
/etc/auto_direct – Contains details of mount-point and its source
/etc/cron – Daemon responsible for cron jobs
/etc/cron.d – Contains allow & deny files of cron jobs
/etc/default – Contains all the default settings of the server
/etc/defaultdomain – Contains the domain-name of the system
/etc/dfs – Contains three files pertaining to sharable files on the server
/etc/ftpusers – This is the file which authenticates FTP sessions
/etc/mnttab – This is the file which acts as input for mount. This is a non-editable file
/etc/motd – Contains message of the day
/etc/mkfs – This creates file systems
/etc/named.boot – Contains details of the DNS server ( server side info)
/etc/nfs – This is the log file of NFS configurations
/etc/opt – Contains third party software configurations
/etc/rmount.conf – This is used to customise the process of mounting your drives
/etc/sulogin – This is the database file for su command
/etc/swapadd – This is the swap configuration file
/etc/system – Contains all the parameters of the kernel
/etc/uadmin – For graceful shutdown of the system
/etc/in.named – This is the daemon of DNS server
/etc/dfs/dfstab – Contains details of shared resources
/usr/sbin – Contains all commands used by sysadmin
/proc – Contains list of all running processes
/etc/default/login – It’s a semaphore login where console variable is commented to give root user access to login remotely to server
/etc/hosts.equiv – This is the file to be edited when quivalent users exist in both systems for trusted relationships.
/.rchosts – Contains the trusted user entry on the remote host
/var/admin/sulog – Maintains log informatioin for su command
/etc/named.conf – This is the file which is created by giving named-bootconf command
/var/adm/messages – This is the logfile updated by syslog daemon
/usr/lib/cron – This is also a daemon responsible for cron jobs
/usr/lib/cron.deny – This is the file linked to /etc/cron.d/cron.deny which is the list of users who cannot perform a cron job
/usr/lib/cron.allow – This is the file linked to /etc/cron.d/cron.allow which is the list of users who can perform a cron job. This is linked manually
/var/adm/utmpx – This is the database file “who” and “finger” commands
/etc/nsswitch.conf – This is the configuration file containing information of naming services like DNS, NIS, NIS + etc
/dev/rmt – Contains the location details of the logical names of the raw magnetic tape
/etc/dumpdates – This file is used to compare different backup dates and USF dump updates
/usr/lib/lpadmin Helps to add, delete and configure logical printers
/var/spool/lp/request – Contains list of all pending jobs on the server
/etc/log – This is linked to /var/adm/log which Contains the log files
/usr/lib/lpsched – This is the daemon responsible for print jobs
/usr/lib/share/terminfo – Contains database of printer models
/etc/lp/interfaces – This file stores the names of all the logical printers created
/usr/spool/lp/requests – Contains files which stored the name of print job id’s
/etc/others – Contains all MAC id’s of remote host which must be manually entered (Resolves MAC id’s to hostname)
/config/sysidcfg – Contains the jumpstart client system information
/etc/bootparams – Contains kdmconfig command results for network syste,s while doing custom jumpstart installation
/etc/rc2.d/S99dtlogin – Contains the shell script responsible for desktop login (GUI based)
/var/mail – Contains all the mails sent to users
/bin/passwd – Contains all the commands to assign passwords to users
/etc/oshadow – This is the backup of the shadow file
/usr/lib/nis – Contains all executable commands of NIS+
/var/tmp – Contains the temporary opening files for vi and other editors
/usr/dt/bin/dumail – Contains the commands to send the desktop mail
/usr/lib/sendmail – This is the daemon responsible for all sendmail services
/usr/lib/fs/nfs/nfsfind – Check shared NFS file systems for NFS files that are more than a week old
/dev/diskette – This is the block device used for mounting a floppy
/usr/dt/bin/dtpad – Contains the open text editor for entering text
/var/nis/data – Contains the file here BIS database files are stored
/etc/default/fs – Stores the default file system used to mount
/etc/dfs/fstypes – Stores all information pertaining to different types of file systems
/usr/ucb – Contains the default files given by the university of Berkeley
/etc/apache – Contains the files where log as well as configurations are stored
/usr/apache/bin – Contains all the binary files for apache service
/var/adm/wtmpx – This is the database file for the last command
/usr/proc/bin – Contains all the process management commands
/etc/protocols – Contains all the protocols
/usr/lib/adb/cred – Contains credentials for all users in NIS+
/usr/local/samba/lib – Contains configuration files for samba service
/var/spool/lpoadmin/lp/interfaces – This is linked to /etc/lp/interfaces which stores the name of logical printers
/usr/spool/lp/model/standard – This is the file copied when you create a network wide logical printer
/usr/sadm/admin/bin/printmgr – This is a GUI tool for configuring printers. This also allows you to configure network printers
/var/optSUNWconn/ppp.log – Contains all the log messages sent by pppd daemon (Point-to-Point protocol)
/usr/local/samba/bin/swat – This is the executable file for Samba (web admnistration tool)
/etc/path_to_inst – This is the file which stores the physical names of the devices used by the kernel
/etc/format.dat – This is the data file for the “format” program. This file defines the known disks, disk types and partitions.
/usr/local/samba – Contains the samba configuration files
/etc/dhcp.<interface name> – This is the configuration file used by a DHCP client
/usr/spool/admin/lp/interfaces – This is linked to /etc/lp/interfaces which stores the name of logical printers
/usr/openwin/share/postscrip/demo – Contains the post script images
/usr/spool/cron/crontabs – Contains files of allow and deny files of crontabs
/usr/spool/cron/atjobs – This also Contains files of allow and deny files of crontabs
/var/named/named.hosts – This is the file which helps resolve hostanmes to the IP address of the DNS server
/var/named/named.local – This is the file which stores local information in the DNS server
/var/named/named.rev – This is the file responsible for resolving IP addresses to hostname (in the DNS server).
/dev/dsk – Contains all the block special files
/dev/rdsk – Contains all the character special files
/etc/init.d/volmgt – File responsible for volume management i.e. floppy, CDROM etc.