Linux Filesystem Tree Overview

/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.

/devcontains all device files, which are not regular files but instead refer to various hardware devices on the system, including hard drives.

/etccontains 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.

/libcontains 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.

Examples

An absolute path name, pointing to what is normally an executable file on an Ubuntu system:
/usr/bin/test

An absolute path name, but pointing to a directory instead of a regular file:
/usr/bin/

A relative path name, which will point to /usr/bin/test only if the current directory is /usr/:
bin/test

A relative path name, which will point to /usr/bin/test if the current directory is any directory in /usr/, for instance /usr/share/:
../bin/test

A path name using the special shortcut ~, which refers to the current user's home directory:
~/Desktop/

Path names can contain almost any character, but some characters, such as space, must be escaped in most software, usually by enclosing the name in quotation marks:
"~/Examples/Experience ubuntu.ogg"
or by employing the escape character
\:~/Examples/Experience\ ubuntu.ogg

https://help.ubuntu.com/community/LinuxFilesystemTreeOverview