Did you ever wonder what exactly should be in the /usr/bin directory? Or maybe what goes inside the /usr/local/sbin directory? I found a great resource of the standards for UNIX describing all the directories in a basic UNIX filesystem. Very handy!

Filesystem Hierarchy Standard
The filesystem standard has been designed to be used by Unix distribution developers, package developers, and system implementors. However, it is primarily intended to be a reference and is not a tutorial on how to manage a Unix filesystem or directory hierarchy.

You can find the document here: http://www.pathname.com/fhs/

[robg adds: I haven't read the whole thing, but it looks to be a good overview of generic UNIX filesystem structure. There will clearly be areas where OS X differs, but this should be a nice starting point if you're wondering what all those directories are for.]