Very summarized, it would be like this: KDE is a community of people, who create the following things:

• The KDE Frameworks, a set of libraries and frameworks to build their software, currently at version 5.30.
• The Plasma Desktop, currently at version 5.9.x, based on KDE Frameworks.
• The KDE Applications collection, which include things like Dolphin (file manager) or Okular (document viewer). This collection is currently at version 16.12, and includes applications built on top of KDE Frameworks 5, and a few still based on the older KDElibs 4.x. These applications do not require the Plasma Desktop.

Well, that wasn't so summarized in the end xD