Just to clarify: Ethel makes it out as though Ubuntu is different to Linux. This isn't really the case.
First, name confusion. Linux is popularly the name for two things -- a kernel, and an operating system (incorrect usage). The kernel (all operating systems have a base 'controller', the kernel, that does core tasks like memory management) is called Linux. The operating system is correctly called GNU/Linux, but many call it Linux.
I assume you mean the operating system. As such, it's a complete replacement for Windows. It was created before Windows was, as the starter of it all (a man named Richard Stallman) came to the realisation that non-free software was a bad thing. Here is another term much-confused. He uses free as in 'freedom', not 'doesn't cost anything'. It means that you should have the right to edit the stuff that runs on your own computer, since it's your property. In much the same way that you have the right to play about with your car engine, if you so wish.
Most stuff in GNU/Linux is 'free software'. This means that you can download it and run it as you like, with no license issues or anything like that, which plagues non-free software. It tends to also be free of cost too, but it doesn't have to be like that in general.
Now, there are lots of parts to GNU/Linux. Many different programs all packaged together to make a complete working operating system. As such, these can be packaged in different combinations, and so you end up with different 'distributions' of the GNU/Linux system.
One such 'distribution' is called Ubuntu (what Ethel refers to), and it's actually very good -- very easy to use.