Webkit is an open source browser project. It's based on a rewrite of KDE's browser, mostly done by Apple. KDE was going to merge it back into KHTML, but they found Apple hard to work with (Apple made lots of big changes without explaining them very well, and KDE didn't have the resources to keep up).
It powers Safari and Chrome, and a lot of smaller projects (mobile browsers, email clients, etc).
"Webkit" can refer to the browser (Apple's fork Safari, or a free version of Safari), the rendering engine (WebkitCore), or the rendering and javascript components (WebkitCore and JavaScriptCore).
Google uses WebkitCore (or maybe a fork) for rendering, but their own Javascript engine.
I'm guessing ghost.py uses WebkitCore and JavaScriptCore, so you can find out how a page will be seen by Safari (and probably Chrome, since Google's JS engine shouldn't be that different in the way it behaves).
