Hyrule is the land in which most of Link's adventures take place. It is a vast kingdom, and has terrain which varies from mountains to deserts.
Hyrule has always existed as a blessed land which was specially gifted by the goddesses. It has existed for eons, and has experienced drastic change over time. In fact, there has been more than one Hyrule, as the original land was flooded in The Great Flood.