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KDE is officially a community dedicated to developing FOSS software. It's legally represented by the KDE e.V. and the KDE Free Qt Foundation. The latter is to ensure that the dual-licensed Qt software used by KDE stays open source.
KDE Plasma is the desktop environment made by KDE. It's sometimes called just KDE because, duh, it shorter.
The apps included in Plasma are also made by the KDE (also see KDE Gear).
There's the KDE Frameworks, built on top of Qt.
Plasma mobile is a distribution for mobile devices.
KDE Neon is a "distribution" showcasing the lastest KDE software.