The KWin effects library contains the interfaces for Effects and the EffectsHandler, which is the bridge between KWin core and the Effect System.
The GL Utils are part of the effects library and provide an abstraction on OpenGL for the use in KWin.
The decoration library provides the interfaces to write a window decoration plugin and the bridge between KWin core and the decoration plugin.
Aurorae is a themeable window decoration and serves here as an example for a window decoration plugin implementing the interfaces provided by the decoration library.
The Compositor is actually part of the core and uses some of the classes provided by the gl utils library and implements the bridge between KWin core and the effects.
The Compositor consists of two parts. The first one is the implementation of the interfaces provided as a bridge between core and effect system:
The second one is the actual compositor which renders the window textures to the screen:
KWin's Core is rather monolithic with the big Workspace and Client classes. It is the implementation of the window manager with the hear being the Workspace class.
TabBox is the part of KWin core handling Alt+Tab functionality. It uses a Qt ModelView setup to manage the selection. The view can either be a normal window or be delegated to an effect.
The Tiling classes handle the layout of the windows in the different tiling strategies.
The Wayland support of KWin as used in the kwin-wayland branch.
And here the complete diagram with all the interactions: