It's a bit like the differences in cars. You have a basic functionality and a set of common features that allows people to build accessories. But, many things are different. As with cars, you will not likely look at ALL the choices before making a selection.
The analogy, of course, breaks down rapidly. In Linux, there are many utilities that are pretty much common across all distros. Some of these--developed under the GNU project--actually pre-date the Linux kernel. (The purists a quick to remind us, that we are really using **GNU Linux**).
I think differences in distros fall into two broad classes: features and configuration. Configuration refers only to how the GUI is set up, where config files are stored, etc. Features are substantive differences in capability.
Out of curiousity, why RedHat AND Fedora??