What is WebKit? How is it used by browsers and CSS?

WebKit is a HTML web browser rendering engine which is used by Safari and Chrome browsers.
It supports custom css properties which can be prefixed with -webkit-

For example -webkit-font-size will be replaced by the standard font-size.

Below is the list of various other browsers and rendering engines, css-prefix used by them:

BrowserEngineCSS Prefix
Google ChromeBlink-webkit-
Mozilla FirefoxGecko-moz-
Internet ExplorerTrident