The man pages[1][2][3] for the hier command on various BSDs are also worthwhile reads if you want to familiarize yourself with *NIX directory structures in general.

[1] http://www.freebsd.org/cgi/man.cgi?query=hier

[2] http://www.openbsd.org/cgi-bin/man.cgi?query=hier

[3] http://netbsd.gw.com/cgi-bin/man-cgi?hier