A real quick overview:
* http://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
More in depth:
* http://www.pathname.com/fhs/
e.g. http://linux.die.net/man/7/hier
However, I found these useful too when I was learning it. Sometimes it just helps to read different explanations of the same thing till it sticks:
http://linuxcommand.org/lts0040.php
http://rute.2038bug.com/node20.html.gz
http://www.reddit.com/r/todayilearned/
e.g. http://linux.die.net/man/7/hier