KDE is a comprehensive desktop environment with a huge number of applications written for it and a huge amount of users. It is based on the Qt framework.
The KDE5 system has two main blocks: the libraries, called KDE Frameworks 5 or KF5, which can be used in other environments, and the desktop environment called KDE Plasma 5. Most of the applications written by the KDE team only use KF5, and do not need the plasma environment.
For more information visit the official KDE project site at https://kde.org/.