KDE Plasma is a cross-platform desktop environment and software compilation, commonly used on Linux.
KDE Plasma is an open-source, cross-platform desktop environment and software compilation based on the qt toolkit. It is one of the two most popular desktop environments on Linux, the other being gnome.
KDE originally stood for K Desktop Environment, but it has since been rebranded as just KDE.
For questions specific to the KDE 4 version, you can use the kde-4 tag.