Tipmonkeys has made a summary about different distributions for beginners, so they can have a look at what's feasable for them. Debian, Ubuntu, Mepis and Knoppix are mentioned.
Allthoug Knoppix is mentioned as a minor distribution, there is a great deal about Debian GNU/Linux and Ubuntu – Quote:
The really monumental thing about Debian is the package mangement system. Dpkg is very similar to RPM in allowing pre-built packages to be installed, but has an online system called Apt-get. Apt-get allows the user to simply type apt-get install package_name and it handles pulling not only the package requested, but any necessary dependencies, and helping configure the install. Debian has a HUGE archive of packages, which means a Debian user very rarely would need to acutally track down a package and its dependencies manually.
Enjoy the whole thing here.