Linux filesystem structure is not imposed by the system. filesystem structure in linux Can vary from one system to the other, even between two GNU/Linux installations!

Directory Explanation
/ Root directory
/bin/ Basic, essential system commands
/boot/ Kernel images, initrd and configuration files
/dev/ Files representing devices. /dev/hda: first IDE hard disk
/etc/ System configuration files
/home/ User directories
/lib/ Basic system shared libraries
/lost+found Corrupt files the system tried to recover
/media Mount points for removable media:
$ /media/usbdisk
$ /media/cdrom
/mnt/ Mount points for temporarily mounted filesystems
/opt/ Specific tools installed by the sysadmin.
often used instead
/proc/ Access to system information.
$ /proc/cpuinfo
$ /proc/version
$ ...
/root/ root user home directory
/sbin/ Administrator-only commands
/sys/ System and device controls (cpu frequency, device power, etc.)
/tmp/ Temporary files
/usr/ Regular user tools (not essential to the system)
$ /usr/bin/
$ /usr/lib/
$ /usr/sbin
$ ...
/usr/local/ Specific software installed by the sysadmin (often preferred to /opt/)
/var/ Data used by the system or system servers
$ /var/log/
$ /var/spool/mail
(incoming mail)
$ /var/spool/lpd
(print jobs)...

The Unix filesystem structure is defined by the Filesystem Hierarchy Standard (FHS)