Filesystem Hierarchy Standard (FHS)

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A FHS standard compatible root has the following structure:
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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

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/usr hierarchy
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bin	Most user commands
include	Header files included by C programs
lib	Libraries
local	Local hierarchy (empty after main installation)
sbin	Non-vital system binaries
share	Architecture-independent data

Reference:
http://proton.pathname.com/fhs/pub/fhs-2.3.html