I'm not a Linux expert by any means, have only fooled around with it, but I can help a little
Distro = a version, or distribution, of Linux. Ubuntu, Debian, Slax are three examples of distros. Some are very similar and some are wildly different to each other, they may vary in the included software, driver/hardware support, default desktop environment and theme, or many other items.
Debian, Ubuntu and Kubuntu are all distributions of Linux, some of the better known ones in fact. Kubuntu and Ubuntu are quite closely related.
I'll leave it for others who are more experienced to give you a better explanation but hopefully that's of some help?