css round corner what is -moz-, -webkit-

[html]
.box {
  -moz-border-radius: 15px;
  -webkit-border-radius: 15px;
  border-radius: 15px;
 
  -moz-box-shadow: 10px 10px 20px #000;
  -webkit-box-shadow: 10px 10px 20px #000;
  box-shadow: 10px 10px 20px #000;
}
[/html]

-moz- –> firefox 1.0
-webkit- –> safari 0.2 and chrome 3.0
border-radius –> opera 10.5, IE9, Firefox, Chrome

more detail in developer.mozilla