James Barnett39,198 Points
-webkit is a vendor prefix it refers to CSS property that only works in Chrome, Safari, Android (and recently Opera) browsers.
You can read all about vendor prefixes here: http://webdesign.about.com/od/css/a/css-vendor-prefixes.htm
-webkit is a "vendor prefix" for the webkit based browsers. It is prefixed before a CSS property that hasn't been fully accepted by all browsers. For example, -webkit-transform property is used for the new 3D transform functionality being made available via CSS. "Vendor prefixes" are also available for microsoft (-ms), mozilla firefox (-mozilla) and others. A good resource for determining if a vendor prefix is needed is http://caniuse.com
Hope this helps.