Kubuntu

In short: Kubuntu is the official Ubuntu flavour with the KDE Plasma desktop environment.

In more detail: It uses the same packages as Ubuntu, but the customisable Plasma desktop instead of GNOME.

In Depth

Details

The desktop looks classic and is highly configurable (widgets, themes). Popular with people switching from Windows. See Debian, Linux.

See also: Linux, Debian, Ubuntu, Fedora