Structure

The Linux File System Structure

All users have a home directory and every named home directory is contained in a directory called "home" that is at the root level of the filing system. There are other directories at this level and they serve certain purposes as described below.

This command will list the directories and files at the root of the file system tree;

ls /
bin  boot  dev  etc  home  lib  lib64  lost+found  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

https://help.ubuntu.com/community/LinuxFilesystemTreeOverview Here, only the most important directories in the system will be presented.

/bin contains the programs which are the commands used in the shell. You can use ls /bin to list its contents and you will see that ls its self is there in the list.
/boot contains files needed to start up the system, including the Linux kernel, a RAM disk image and bootloader configuration files.
/dev contains all device files, which are not regular files but instead refer to various hardware devices on the system, including hard drives.
/etc contains system-global configuration files, which affect the system's behavior for all users.
/home contains named home directories for each user to which they have free access.
/lib contains very important dynamic libraries and kernel modules
/media is intended as a mount point for external devices, such as hard drives or removable media (floppies, CDs, DVDs).
/mnt is also a place for mount points, but dedicated specifically to "temporarily mounted" devices, such as network filesystems.
/opt can be used to store additional software for your system, which is not handled by the package manager.
/proc is a virtual filesystem that provides a mechanism for kernel to send information to processes.
/root is the superuser's home directory, not in /home/ to allow for booting the system even if /home/ is not available.
/run is a tmpfs (temporary file system) available early in the boot process where ephemeral run-time data is stored. Files under this directory are removed or truncated at the beginning of the boot process. (It deprecates various legacy locations such as /var/run, /var/lock, /lib/init/rw in otherwise non-ephemeral directory trees as well as /dev/.* and /dev/shm which are not device files.)
/sbin contains important administrative commands that should generally only be employed by the superuser.
/srv can contain data directories of services such as HTTP (/srv/www/) or FTP.
/sys is a virtual filesystem that can be accessed to set or obtain information about the kernel's view of the system.
/tmp is a place for temporary files used by applications.
/usr contains the majority of user utilities and applications, and partly replicates the root directory structure, containing for instance, among others, /usr/bin/ and /usr/lib.
/var is dedicated to variable data, such as logs, databases, websites, and temporary spool (e-mail etc.) files that persist from one boot to the next. A notable directory it contains is /var/log where system log files are kept.