Re: so many versions
You'll only get downvoted if people think your question is rhetorical.
Your main problem with lacking understanding of "What is Linux" comes down, probably, through lack of experimentation and experience of it.
Linux is, at its core, just a way of bringing together hardware interfaces with software ones. That's the kernel: it manages memory usage, separation of separate "programs", etc. It's all the invisible stuff that you don't think about. On top of that, you can place whatever you want.
Most people place free and open source (FOSS) software on top, which has sort of been known as "Gnu/Linux", since a lot of software that falls into this category and is used is made by Gnu. It was originally intended as a unix clone, but has now grown its own wings and deviated significantly.
A collection of different software and configurations is known as a Linux distribution ("Distro"), of which Ubuntu is one, Fedora is another, Red Hat is a 3rd, Debian, Gentoo, etc. They're all just subtly different ways of pulling together a whole system, which is useable by someone like you to run things like a Web Browser or Office program.
The main topic of this article was "Window Managers". These are what you would recognise as "The Desktop + Start Menu". There are a few of these about, the main traditional ones being KDE and Gnome. Basically, different people have different preferences, so they prefer different Window Managers. If you think about the difference between Windows 7 and Windows 8, the main differences that you see will be in the Window Manager used. Android could be argued to be another window manager, designed primarily for mobile application.
The best way to find what you like is to install a couple of versions (probably in a virtual environment for ease) and give them a go. You can even run many different versions from USB stick; no installation required!