/bin – This directory contains most non-privileged system commands such as ls, mkdir, rm, etc.
/boot – Contains the systems boot image, boot loader, and the kernel
/dev – Contains Symbolic links to system devices such as optical and removable drives
/etc – Contains all system configuration files and most configurations for installed packages
/home – Contains a directory for each user and contains profile information
/lib – Contains dynamic libraries and modules for the Linux system and installed packages
/media – Contains mount points for optical drives and removable media
/mnt – Used as a location for mounted drives and shares
/opt – Contains user installed packages and custom software which are not handled by the system or package manager
/proc – An interface between the kernel and the system, useful for system diagnosis information
/root – The superuser (root) home directory
/sbin – Contains privileged commands that are usually run as superuser (root/sudo)
/sys – An interface between the kernel and the system, used for modifying system settings
/tmp – A location for temporary files such as sessions on a web server
/usr – Contains most installed packages that are not part of the system, user installed programs
/usr/bin – Contains commands related to user installed packages in /usr
/usr/sbin – Contains privileged commands related to user installed packages in /usr
/var – Contains files that change often or accessed frequently
/var/log – Contains all system logs and most logs generated by installed packages which are very useful in diagnosing system issues ( a must know for system admins)