Slackware is one of the first Linux distributions. It's first release was in April of 1993 and since then the author, Patrick Volkerding has aimed at producing a simple, stable, "UNIX-like" Linux distribution.
Slackware aims to comply with published Linux standards. This helps you to learn Linux, not Slackware. It involves some manual editing of well commented config files and few GUI config tools. The refreshingly simple package management is based on standard gzipped tar archives and offers no dependency checking, expecting the user to resolve dependecies manually.