Slackware is the oldest Linux distribution properly-speaking in existence. Basically Slackware is quite synominous with linux. There is a glorgied quote that gets dropped in the oh so helpful slackware community that goes, "If you learn Red Hat, you learn Red Hat. If you learn SuSE, you learn SuSE. But if you learn Slackware, you learn Linux." It is very true as well.
I think to be more accurate, if you know the kernel (GNU/Linux) you know Linux.