Ubuntu and Kubuntu
Ubuntu and Kubuntu are based on Debian these are the main two from the Canonical company, Debian is the one of the oldest and probably the most open source versions out there. But it also probably has one of the biggest problems in that it is community driven which in turn means that unless they are not happy with something it can take a long while before something gets applied.
But in steps Ubuntu, which is based on Debian, and basically they are doing development and also with release cycles or every 6 months with LTS (Long Term Support) as well, this has helped Debian at the base. The Ubuntu release is very polished and also very similar to other operating systems, Ubuntu users use a interface called Gnome and Kubuntu uses the GUI interface KDE both are very nice GUI interfaces and it is all down to what you prefer since it is all FREE!. I personally prefer the KDE interface probably because it is more similar to Windows (which is not a bad thing at times since they are spent all of time researching into GUI interfaces), and it is very intuitive.
Ubuntu,Kubuntu, Debian etc, are all distributions of Linux (the main operating system, that talks to the hardware) with different user interfaces, it is mainly all open source, if there is some propriety applications/firmware that has to be installed you can either click a button if there is a question asking you do you agree to the terms of installing this sort of software (there is many wars in the open source world about installing propriety software onto open source software, I personally think if it makes it easier/or work then what is the problem). Or you may have to find a forum/website that instructs you how to install.
The main thing is choice, and if you want to check it out without installing onto your hard drive which for newbies can be a hard thing to think about you can use a virtual machine, which I personally use as well to test out different operating systems, the one that I use is called virtualbox and it does the job very well.