Webkit is basically an open source layout engine that the web browser uses to render the HTML/CSS web pages. Currently, among the desktop browsers, WebKit is used by Safari and Google Chrome. In the mobile/tablet space, WebKit is used by the default browsers of iOS, Android, Blackberry Tablet OS, webOS and also by the browser in Amazon Kindle. It also currently has the most market share among all layout engines.
Here is a list of the layout engines used by the most common desktop browsers along with the CSS prefixes:
Safari & Chrome
Another thing to note with regards to the CSS prefixes is that a layout engine will only recognize and parse it’s corresponding CSS prefix, so if Gecko sees a -webkit prefix, it would ignore it.