The idea is simple enough: Pick the sentence that describes you and your needs most closely, and look at the distributions mentioned there. The distributions are listed in roughly descending order of appropriateness. If you don't see a distribution mentioned there, it may just be that I was trying to keep things simple; there may be room for it in a future edition.
Rationale: These distributions seem to have the best mix of general-purpose applications (or access to same) served up in a in a relatively user-friendly wrapper. I've grouped Red Hat and Fedora together since Fedora is a derivative of the Red Hat line; Fedora is the more consumer friendly version of the two and comes with only community-level support, while Red Hat is aimed at businesses and professionals and comes with support from Red Hat Inc. itself.
Rationale: Both Red Hat and Ubuntu have relatively easy-to-administer server editions of their distributions. In theory, most any Linux distribution can be a server -- there's no real distinction between a "desktop" or "server" edition of the product as there is with Windows -- but it helps to have a distribution with the right tool mix out of the box.
Rationale: There's no better way to learn Linux than by working with a distribution that demands that you learn as you go. These distributions are not as user-friendly, but you'll definitely learn.
Rationale: These smaller distributions are built to run most anywhere, and are a good way to quickly determine if an older computer can be kept running.
There may be other ways to break this down, but to me the most effective way to do it is to ask: What do you want to do? If you have that in mind, it's a lot easier to pick the right distribution than almost any other way I can think of.