I just came across of Filesystem Hierarchy Standard (FHS). It is a standard that consists of a set of requirements and guidelines for file and directory placement under UNIX-like operating systems.
The guidelines are intended to support interoperability of applications, system administration tools, development tools, and scripts as well as greater uniformity of documentation for these systems.
The following directories, or symbolic links to directories, are required in /.
Directory Description /bin Essential command binaries /boot Static files of the boot loader /dev Device files /etc Host-specific system configuration /lib Essential shared libraries and kernel modules /media Mount point for removeable media /mnt Mount point for mounting a filesystem temporarily /opt Add-on application software packages /sbin Essential system binaries /srv Data for services provided by this system /tmp Temporary files /usr Secondary hierarchy /var Variable data
Official site http://www.pathname.com/fhs/