Unix was and is (mostly) proprietary. So some people got their collective noggin together in the 1980s and put together something that reproduced Unix functionality without copying its source code, which would have been illegal [insert longer version about Richard Stallman and Linus Torvalds here].

One of the first GNU/Linux distributions was Debian. Ubuntu was created by people that had been involved with Debian and Ubuntu is officially proud of its Debian roots. It's all ultimately GNU/Linux but Ubuntu is a flavor. In the same way that you can have different dialects of English. The source is open so anyone can create their own version of it.

However, Debian is not really useable for mere mortals such as myself and that is where something like Ubuntu comes in. Indeed, I understand that it is specifically the strategy of Canonical, the company behind Ubuntu, to target the non-geek user. Which is why most new GNU/Linux users come to Ubuntu or Mint (which is based on Ubuntu) first.