Debian, Suse, Fedora etc. all have similarities, so understanding and learning one also means most of your new skills can be applied to allof the other distributions. Debian has the reputation of being stable; however, it also is a little harder (or less user friendly ) than say Suse . It also does not offer the latest packages, like Fedora.
That said they are all based on the same building block - the Linux Kernel. If you understand the Kerenl principles and the boot loader sequence your half way their to understanding all of the distro's.
If you're interest in learning more about Debian their Wiki has lots of information.