Slackware is the oldest LinuxDistribution still around. The philosophy it follows is that the SysAdmin knows what they're doing and doesn't need handholding. It aims to be as close to Unix as possible while complying with the published Linux standards. That may result in some legwork when setting up a system, but the installation is just about unbreakable. Anyone with rudimentary Linux knowledge should be able to install Slackware without any trouble.
The Package format is as simple as it gets - plain old TarBalls of the files to install, plus an installation script. Slackware itself offers only rudimentary PackageManagementTools that mainly aid upgrading or uninstalling. However, a variety of third party PackageManagementTools that include dependency resolution now exist, several of which are endorsed by Slackware.
Installation of software not included in the current or latest version of the distribution usually means compiling from source, though a third party package repository exists at 
Linuxpackages.net. You definitely want to have a look at checkinstall, a thiry party utility which monitors make install (or any other installation process) and assembles the copied files into a package, making later removal sane and simple.
The current version of the distribution is 10.0. It comes on two CDs chock full of goodies. The second contains mainly KDE and GNOME, all the rest is on the first disc.
See also
The Slackware Linux ProjectSlackware Linux Essentials - The Official Guide To Slackware LinuxLinuxpackages.net third party package repositoryPart of CategoryDistribution
