As you probably know, Ubuntu (as most Linux distributions) are made up out of many packages. When you install a distribution, not every package available is installed, but a certain selection of packages.
The different "flavours" of Ubuntu (Xubuntu, Kubuntu, Lubuntu and others) basically differ in what that initial selection of packages contains. Ubuntu by default installs for example the Gnome Shell desktop environment, Kubuntu installs the KDE desktop environment and so forth.
But once you've installed the distribution, you can basically "mix and match" like you did when you installed the
xubuntu-desktop package. Now, your system offers you multiple desktop environments or window managers to choose from when you log in.
As for what your system technically is, it's an Ubuntu system with some additional packages installed that would by default be installed on a Xubuntu system. What exactly to call that is in the eyes of the beholder ;)