The Linux File System
/bin - Binaries
The /bin folder contains the basic set of binaries available to every user. It contains the binaries for basic commands such as ls, cat, chmod, mv, and more.
/boot - Bootloaders
The /boot folder contains the files necessary to load the operating system when the computer is turned on.
/dev - Devices
The /dev folder contains files that represent devices. In Unix-like systems like Linux, disks and just about everything else are represented as files.
For example: sda is a disk. Originally sd stood for SCSI device but became a catch-all for any block storage device. The letter that follows sd, a in this case, represents the order in which the device was found.
/etc - Etcetera (and so on)
The /etc folder contains system-wide (as opposed to user-specific) configuration files.
/home - Home Folders
The /home directory contains home directories for users other than root.
/media - Media
The /media folder is where removable disks (e.g. CDs, thumb drives) are automatically mounted.
/mnt - Mount
The /media folder is where removable disks (e.g. CDs, thumb drives) are manually mounted, very similar to the /media folder.
/opt - Optional
The /opt folder is intended to hold manually installed 3rd-party software.
/proc - Processes
The /proc folder contains pseudo files that contain information about processes and resources. All running processes have a folder under /proc.
/root - Root (user) Home
The /root folder is the root user's home directory
/run - Run (temporary) Files
The /run directory contains temporary files, all of which actually reside in memory. The contents of this directory are deleted upon reboot (because memory is cleared).
/sbin - System (or Super User) Binaries
The /sbin folder contains binaries used for system administration tasks including disk and network management. It contains the binaries for commands such as chkconfig, ifcondfig, and fsck.
/srv - Service (or Server) Data
The /srv folder stores service/server data to be accessed by external users, for example via a web server.
/sys - System Temporary Files
The /sys folder contains temporary files used by the system, all of which are stored in memory (very similar to the /run folder). The contents of this directory are deleted upon reboot (because memory is cleared).
/tmp - Temporary Files
The /tmp folder contains temporary files. Unlike the /run folder, files stored in /tmp are less restricted and may be accessed by other users. Generally speaking, developers should prever to use /run over /tmp for security.
/usr - Users
The /usr folder contains "non essential" apps and files used by users (as opposed to the system).
/var - Variable
The /var folder contains files that are expected to grow over time (e.g. logs and caches).
Deeper Knowledge on Linux File System
chmod Linux Command
A Linux command used to change the mode of a file or directory
Broader Topics Related to Linux File System
Linux
A free and open-source operating system