Ubuntu is a Linux distribution that focuses on being easy to use, particularly for people who are switching from Windows. Linux, in case you are unfamiliar with it, is an operating system, much like Microsoft Windows, or Apple’s OSX.
The difference between Linux and many other operating systems is one of both cost (You can get most versions of Linux for free) but, perhaps more importantly,the fact that it is open-source. What this means is that the code used to write Linux is freely availible to look at and modify, and many peopel do. This means Linux isn’t made by any particular company, but rather by a community of people (some of which are companies). The different distos (Distributions) of Linux usually determine how it is set-up, what software is included with it, how much support you’ll get, etc.