​Source is basically made up of the following:

​JavaScriptCore

​Web Template Framework

WTF is an internal library, which contains various utilities used as part of the JavaScriptCore and WebCore. Example of such classes are ​Vector, ​HashMap, ​RefCounted (classes to that implements multiple ownership), ​RefPtr, etc...

​YARR! or Yet Another Regex Runtime

This is a faster replacement for our regexp engine.

JavaScriptCore

The JavaScript engine. 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.

JSC is split in different directories inside the directory. Those directories include ​dfg, ​assembler and ​jit.

The ​API directory contains the JavaScriptCore C API.

​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)