Jason is correct....
Jason is correct.
Linux OS always has Linux kernel inside. They don't need any other software to be called Linux. And if it doesn't have that, it cannot be called Linux.
Biggest similarities user will see usually are package management system and desktop environment. Distributions could easily be divided into Gnome-distributions, KDE-distributions and several others.
Also they could be divided by package-management system like ones using .deb, rpm or source or others. There are also rolling-releases and normal relases. rolling-release distributions like Gentoo always have updated software, but they may not be as stable and well tested. Fixed releases like Debian may have software that's a couple of years old. They still get security updates so it's not a problem.