Here, only the most important directories in the system will be presented.
/bin is a place for most commonly used terminal commands, like ls, mount, rm, etc.
/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 home sweet home, this is the place for users' home directories.
/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 addition 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.
/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 variable data that potentially changes rapidly; a notable directory it contains is /var/log where system log files are kept.