KDE is a combination of multiple different parts of which Plasma is one (and also the name of the overall organization that produces the whole thing). Plasma is the main component, its main contribution to KDE is the various user facing components like the desktop, the launcher bar, etc.
KDE is an organization developing Linux desktop software and Plasma is their window manager
But I thought kwin was the window manager. And that the DE in KDE stands for Desktop Environment, so it's weird that it's actually an organisation.