Buon giorno from 2 degrees pitch black wetherby UK :-)

I have just added curved corners to my tutorial website http://tutorial.davidclick.com/curvy.html but i would like please an explanation to why I need to reference moz & web kit border radius as in:

-webkit-border-radius:10px;
-moz-border-radius:10px;
border-radius:10px;

So my question is please why does webkit & moz really do or put another way why does just border-radius do the job?

Grazie tanto
David