The directory hierarchy
The root filesystem (/):
Must contain utilities and files sufficient to boot the operating system, including the ability to mount other filesystems.
Should contain the utilities needed by the system administrator to repair or restore a damaged system.
Should be relatively small.
/usr contains system utilities and programs that do not appear in the / (root) filesystem. It includes directories such as bin, lib, local, and src.
/var contains varying data such as printer spools and logfiles, including directories such as log, mail, and spool.