Slackware was one of the earliest Linux distributions, and is the oldest distribution still being maintained. It was created by Patrick Volkerding of Slackware Linux, Inc. It has a policy of incorporating only stable releases of applications, standing mainly for stability and ease of use. For a good while, other Linux distributions that came after it were in fact evaluated for their "Slackware compatibility."
Slackware uses BSD style init scripts, while most other Linux distros use System V style init scripts. Basically, with System V style each runlevel is given a subdirectory for init scripts, whereas BSD style gives a single init script to each runlevel. BSD style advocates say that it is better because with this system it is much easier to find, read, edit, and maintain the scripts. System V advocates say that the System V structure for the scripts makes them more powerful and flexible.
It is worth noting that System V init compatibility has been incorporated into Slackware, starting with version 7.0.
Source: Wikipedia