Why are so many dirs used in solaris hierarchy?

Hi all,

I would like to know the difference between the different dir structures present in solaris!!!

Meaning what does /usr contain, /etc ,/opt/ ,so on...
I know what /usr and /etc are used for.
But why are /opt /bin /sbin /var and many more that i have missed

I would appreciate if anyone can direct me to a link where i can have extensive study of all these dirs.

thanks

You can check out these..
Solaris Advanced User's Guide
2. Where Things Are (19-Mar-1996)