​Source is basically made up of the following:

​JavaScriptCore

The JavaScript engine. This also includes the WTF library, which contains various utilities. This is completely cross-platform (though there are various platform-specific JIT backends). This can be used as a separate entity from the rest of WebKit.

​WebCore

This is where most of the magic lives. All of the rendering logic, SVG support, transforms, network support, etc. is here.

Each platform has specific logic here. Look in WebCore platform for the OS-specific and rendering (Skia, CG, Cairo) backends.

​WebKit

The front-end layer used by applications. Each GUI platform (windows, Cocoa, Gtk+, Qt) provides a different implementation of this. Generally, the goal is to make this a client to the WebCore layer, basically just 'shimming' WebCore to the specific Os/Platform that will be using WebKit.

The remaining projects are autogenerated bindings to the DOM, etc.

(Taken from ​Brent's email to webkit-help)

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The WebKit source tree also contains the following project:

​JavaScriptGlue

This is a legacy component needed for compatibility with some older Mac OS X software. It will eventually be retired and should not receive any new development.

(Information taken from ​a Bugzilla comment by Darin)