Kubuntu is an official "flavor" of Ubuntu that uses the KDE Plasma desktop environment. This gives it a completely different look and feel.
Kubuntu was sponsored by Canonical Ltd. until 2012, and then directly by Blue Systems. Now employees of Blue Systems contribute upstream, to KDE and Debian, and Kubuntu development is led by community contributors. During the changeover, Kubuntu retained the use of Ubuntu project servers and existing developers.