KDesktop was the component of the K Desktop Environment 3 and earlier, that provided a virtual background window to draw icons or other graphics on. In conjunction with Kicker and Superkaramba it constituted the graphical shell.
In KDE Software Compilation 4, Kicker, KDesktop, and SuperKaramba were replaced by KDE Plasma 4. The graphical shells KDE Plasma 4 and KDE Plasma 5 being widget engines of their own, SuperKaramba is no longer necessary and e.g. "Kicker" was re-implemented as a desktop widget.