Linux (capital "L") refers specifically to the linux kernel.
Debian, Ubuntu, Fedora, Redhat are all linux OS distributions that contain the above kernel as well as a host of other userland tools, configurations, etc.
More specifically, Ubuntu as a distribution, is in a way a descendant of Debian. Package management, configuration, file structure, etc, are very similar on Ubuntu and Debian.
Likewise, Fedora is a descendant of Redhat.
Oftentimes, you hear Linux referred to as GNU/Linux. This is a nod to the fact that Linux itself is only the kernel, and as such is pretty useless in and of itself. To give functionality to that kernel, you need the GNU (and other non-GNU in some cases) userland tools.