You're confusing Kubuntu, a version of Ubuntu that used the K desktop environment, with Xubuntu, a version that uses the extremely fast xfce environment.
As for your main question, .Kencs, a Linux operating system is not one thing, it's a collection of hundreds or thousands of individual packages that are lashed together with an installer to form a Linux distribution. There are hundreds of distros, but you can concern yourself with only the big players. I recommend Ubuntu, it's what I use. You might also want to try OpenSuSE.