The root directory of the system is denoted as `/'. Some of the common sub-directories in the root directory are: home, bin, sbin, usr, etc, var, dev, lib, proc, boot and tmp.
Suppose we have a user named jhack. Let us trace the path to the home directory of jhack. Under the home directory, suppose we have a sub-directory called students. Underneath the students directory, suppose we have the directory jhack. Then the absolute pathname to the home directory of jhack is /home/students/jhack.
The directory /bin (short for binary) contains executable programs commonly used by all users. Look at the programs in that directory. If you are curious about any particular program, then read the man page for that program. The directory sbin contains programs used for system administration.
The directory /lib contains shared libraries and drivers. The directory /var contains variable data used by several system programs. The directory boot has some basic programs that hep in booting up the system. The directory /mnt contains mount points for mounting a file system temporarily. For example, this is where your CDs will show up (as /mnt/cdrom).
The directory /etc contains system setup information. For example, it contains the file passwd that contains the login information about all the users in the system. Under the usr directory, there are are many important system directories. For example, the system man pages are kept in the directory /usr/share/man.
The directory /tmp (short for temporary) is a directory in which any user can write. You can use this directory as a place for storing files temporarily. Usually the /tmp directory has much more space than your home directory.