KWin, the standard KDE window manager in KDE4.0, ships with the first version of built-in support for compositing, making it also to compositing manager. This allows KWin to provide advanced graphical effects, similar to Compiz, while also providing all the features from previous KDE releases. Unlike Compiz, KWin still functions even when not system support for compositing is available, with only the compositing features being unavailable.

More Here