What's with the "vendor prefixes"?

• Vendor Prefixes

.rounded {
    border-radius: 20px 20px 20px 20px;
    -moz-border-radius: 20px 20px 20px 20px;
    -webkit-border-radius: 20px 20px 20px 20px;
    border: 3px solid #000000;
}