From the Preface:

Slackware is the oldest Linux distribution still in active development. Dating back to 1993, it is still being maintained by its creator Patrick Volkerding with help from developers from all around the world.

It is a very sturdy yet simple distribution, whose main goals are simplicity and stability. Slackware also strives to be the most "UNIX-like" Linux distribution.

If you want a distribution that will force you to understand Linux and its Unix heritage, Slackware is for you. If, instead, you want the complexities of Linux hidden from you, look for a different distribution.

Don't however let this intimidate you, Slackware is no more difficult to learn or become proficient in than any other distribution. The main difference will be in the way you perform tasks, Slackware is far more dependant on the command line interface (cli) than it is on graphical tools. If you compare it to driving a car, you can think of Slackware as a manual/stick and distributions like Mandrake and Redhat as automatics.

Intended Audience:

The first section of the Handbook will guide the new user through the installation of Slackware Linux and gently introduces the concepts and conventions that underpin the OS. Working through this section requires little more than the desire to learn and the fortitude to attempt an installation.

The second, far larger, section of the Handbook is a comprehensive reference to all manner of topics of interest to Slackware Linux system administrators. Some of these chapters may recommend that you do some prior reading, and this is noted in the synopsis at the beginning of each chapter.