RedHats installation is well known for being user friendly. I guess it's the easiest system to install when your a novice. RedHat is also the most commercialized system. Since it's commercialized, it is also technically supported. It is an intricate part of the Linux system.
Packages are how the programs are distributed.
Slackware relies on sources. The RPM system makes it much easier to install a program or set of files, but in doing so you loose a certain amount of control over your system. Which, in my opinion, is the reason you went to Linux in the first place, control. I'm not bashing RPM's, I just don't use them. Redhat is also capable of using sources also.