This is the Crux...
Linux is a Kernel. This is what schedules memory management and resource allocation. GNU is the software bundle/packages that come with the distro, hence you often see Debian referred to as GNU/Linux.
All packages share a kernel, it is the other sofware bundle, libraries, installers, etc that are different.
Different distros are also targeted at different skill levels. SCO Caldera, Lycoris, SuSE, Mandrake, Red Hat, etc, are all very easy to install and have good hardware support. However, if you want to *learn* linux and UNIX, these are not going to teach you much. Everything is automagically configured for you.
If you really want to learn, pick something like Slackware (my personal favourite), Crux, Debian, Evil Entity (my other favourite) or Gentoo. You really have to configure most stuff yourself with these. But be warned, do a lot of reading before attempting to install any of these.
Have a look at these website for some more info:
My opinion is if you want something user friendly and good all-rounder go for SuSE. If you want to genuinely learn UNIX, go for Slackware as it isn't really that hard to install. Debian is also very good, but hard to install for a newbie.
Mandrake is really a pumped-up version of Red Hat and is now bankrupt, so I would't go with Mandrake.
All the best.