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.
To Quote the Slackware Web page:
What is Slackware Linux?
"The Official Release of Slackware Linux by Patrick Volkerding is an advanced Linux operating system, designed with the twin goals of ease of use and stability as top priorities. Including the latest popular software while retaining a sense of tradition, providing simplicity and ease of use alongside flexibility and power, Slackware brings the best of all worlds to the table.
"Originally developed by Linus Torvalds in 1991, the UNIX(tm)-like Linux operating system now benefits from the contributions of millions of users and developers around the world. Slackware Linux provides new and experienced users alike with a fully-featured system, equipped to serve in any capacity from desktop workstation to machine-room server. Web, ftp, and email servers are ready to go out of the box, as are a wide selection of popular desktop environments. A full range of development tools, editors, and current libraries is included for users who wish to develop or compile additional software."