Kubuntu is a community developed and supported project. Since its launch in October 2004, Kubuntu has become one of the most highly regarded Linux distributions with millions of users around the world. Our mission is to provide the latest stable KDE software, which includes the flagship project Plasma, our desktop environment on top of a stable Ubuntu core. We release in 6 monthly cycles, following the same freezes/milestones as Ubuntu. https://kubuntu.org/about-us/
Kubuntu is an official flavor of the Ubuntu operating system which uses the KDE Plasma Desktop. As part of the Ubuntu project, Kubuntu uses the same underlying systems. Every package in Kubuntu shares the same repositories as Ubuntu, and it is released regularly on the same schedule as Ubuntu. Simply put, it is Ubuntu with the KDE Plasma desktop rather than the default GNOME desktop.
The Kubuntu team customizes the KDE Plasma desktop environment to provide one of, if not the best “out of the box” experiences. They have an active community so getting help is never a problem and their development team does an excellent job of addressing issues and adding features. If you want to try KDE Plasma, this is where I suggest you start.