Differences from Ubuntu
A regular installation of Ubuntu uses the GNOME desktop, GTK+/GNOME applications and packages (e.g. Evolution), and GTK+/GNOME-based admin tools (e.g. Synaptic Package Manager). In contrast, a regular installation of Kubuntu uses KDE's plasma desktop, KDE/Qt applications and packages (e.g. Kontact), and KDE/Qt admin tools (e.g. KPackageKit). Nevertheless, Kubuntu users can install and use any GNOME/GTK-based package that is available in the Ubuntu repositories, and Ubuntu/GNOME users similarly can install and use any KDE/Qt-based package. One may install Ubuntu and then add the KDE Software Compilation, for example, or can install the entire Kubuntu desktop 'metapackage' on top of Ubuntu. (There is a slight disadvantage to installing and using both the GNOME desktop and the KDE plasma desktop, however, namely the increased diskspace and memory requirements of running both GTK/GNOME and Qt/KDE.)
The Kubuntu desktop is fully customizable. Originally designed to ease transition for users from other operating systems (such as Microsoft Windows) by allowing a similar desktop layout, the KDE Plasma Desktop incorporates widget-centric modularity that allows the user to incorporate function similar to all other operating systems and also create new functionality not found in other operating system desktops. 3-D effects are available in the standard KDE SC 4 installation.
Source: Kubuntu - Wikipedia