Simply put, Ubuntu Linux is a Linux Distribution created by the folks at Canonical, in South Africa.
Most Linux distributions use the Linux Kernel as the base operating system. The creation and maintenance of the Linux Kernel is handled by the Linux Kernel Group ???
There are literally 30 to 40 Linux distributions but Ubuntu has been (and continues to be) the top distribution for the last 2 to 3 years. Basically the folks at Canonical package the Linux Kernel and a lot of other Linux programs into a package that can be installed on various flavors of computer hardware.
Ubuntu Linux is the name most often associated with Canonical's Linux Distribution but there are actuality several flavors that are created by Canonical, such as
Ubuntu, Kubuntu and Xubuntu derive their names from the Desktop Windows Management software they use.
Those familiar with MS Windows know there is only one Desktop Presentation. That is, there is one graphical Interface provided with a Windows installation and that graphical interface is fixed and controlled by the MS Developers.
However, Ubuntu Linux allows you to have several different Desktop Presentation flavors, such as:
The above Desktop Interfaces are not created by Canonical but are actually products in their own right. However, most of the Linux Distributions use the X Windows System (created at MIT) as the base Windows Management System. X Windows is produced by the Xorg folks and is a product in it's own right.
Ubuntu Linux uses the Gnome Windows management subsystem to provide a Desktop Interface to users.