'Zone' system query
 

Greets all and a merry and safe christmas to all of you.

I have noticed in several posts the the term Zone 9 or Zone 5 etc is
mentioned when describing locality. Being from Australia, I have no idea
what this means exactly. Could someone point me to a site or resource that
explains this Zoning system as it would further clarify much of the advice
given in this fine newsgroup.

Thanks.