Swift is a Webkit based browser for Windows (the first of its kind). What does that mean to you? Well, Webkit is the HTML engine that drives Safari, and is included in OS X with lots of hooks for developers. Chances are if you are doing web browsing on the Mac you're using Webkit (though the Firefox based browsers do not make use of Webkit).
Thanks to the open source nature of Webkit Swift is being developed to bring some Webkit love to the PC. I tried out Swift and it is definitely a work in progress but it is something to keep your eye on.