When you tell someone, "I'm going to buy a car," you usually hear, "Which one?" Ford, Toyota, Mitsubishi, Honda, Saturn? Two-door, four-door, minivan? And so on.
Likewise, if you say "I want to run Linux," you'll get the same question: Which one? There's no one "Linux" in the same sense that there's no one "car." There are things common among all cars as there are among all Linux distributions: All cars have an engine, and all Linux distributions share the Linux kernel and many of the GNU utilities.
For most people, Ubuntu is the Linux distribution, and the simplest place to get started.
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But that's where the similarities begin and end. There are easily hundreds, if not thousands, of Linux distributions that are maintained and updated regularly, many of which are tailored for extremely specific needs.
Think I'm kidding about speciialization? There's even a flavor of Linux dedicated to the Asian strategy game Go, called Hikarunix.
Who's to say what's worth running? With such a bewildering variety of choices, it's tempting to simply opt for the familiarity of Windows or even the Mac, both of which come in only a couple of basic variations.
The truth is, you don't need to drive yourself up one wall and down another choosing a distribution. Odds are you don't have to pick from more than one of three or four, tops, depending on your requirements and inclinations.
Rather than just run down a catalog of distributions (something you can easily get at Distrowatch), I've taken about 20 of the most popular and useful distros and grouped them by the way they answer specific user needs. If you read the header of a given section (e.g., "I want something simple for an older machine") and find yourself nodding in agreement, chances are the distributions discussed there will be what you need.