Autodidact

An autodidact is someone who is largely or primarily self-educated. Autodidacticism is a good way to make yourself a better and more well-rounded person, and there's an immense library of books and other learning materials to cater to people who don't have the time or money to take classes, on every imaginable subject. However, one major disadvantage self-teaching has, compared to traditional education, is that it's possible to hyperfocus and avoid viewpoints that go against one's prejudices; as a result, some of the more utterly batshit kooks are self-proclaimed autodidacts.

Autodidacticism is similar to, but not exactly the same as, self-help; in general, the former refers to study of things that would generally come under traditional education, hobbies, or vocational training, while the latter mostly refers to psychology and relationship issues. However, the distinction is not terribly clear and it's safe to say there's a lot of overlap.

The autodidact as folk hero
The long American tradition of rugged individualism has helped to make the lone-wolf intellectual a sort of stock folk hero. While most if not all real innovation works on a "shoulders of giants" principle, the perception is that outsiders have a unique perspective that the Establishment is incapable of seeing. The result is that in certain quarters, scientists straddling the border between mainstream and fringe like Nikola Tesla and Edward Teller are lionized beyond their deserved influence and looked to as role models by would-be scientific revolutionaries, and among the anti-establishment outsiders, fraud, incompetence, and a surreal degree of intellectual laziness abound.

Autodidacts (especially the "boy genius" type) are popular characters on television and in movies, to the point of being a bit of a stock character in some genres of TV; examples include Dr. Spencer Reid from Criminal Minds (who is, however, also a traditionally-educated PhD, albeit very young for his level of accomplishment), Mike Ross from Suits, and Charlie Eppes from Numb3rs.

Credentials and the limits of self-education
There is a meme in certain parts of the libertarian world that claims that professional licensure is a form of legalized restraint of trade. The mythology of the self-taught expert plays into this in a big way, since a lot of cranks have (as described before) painted themselves into an intellectual corner by limiting their exposure to outside ideas and therefore have themselves convinced they know something the experts don't. The result is that autodidactic cranks rail like hell against any perceived barriers to entry, even when (as in the case of a diploma or license) it's generally considered a necessity to prove that the holder has a certain baseline of knowledge to engage the field.

How important this is depends on the field. Pretty much anyone can be a historian, an astronomer, or a programmer with the proper equipment and independent study, but being a medical doctor requires highly specialized, scientifically-validated knowledge. Some fields once had lower barriers to entry than they do now; self-taught lawyers like Abraham Lincoln would have a rough go of it in the modern, highly specialized legal world if they were admitted to the bar at all; modern architects need a knowledge of engineering to deal with building techniques and materials that Frank Lloyd Wright never knew about; and the gap between amateur naturalists and professional biologists has expanded widely as genetics and microbiology became more significant. Similarly, an amateur chemistry set or a hacked-together Farnsworth fusor is a far cry from the specialized lab equipment, clean rooms, and high-energy, multi-billion-dollar particle accelerators that modern chemists and physicists need to do their jobs safely (or at all). Compared with the telescope, camera, and computer that constitutes the entirety of what a good amateur astronomer needs, that's huge.

There are also two issues that self-education has trouble with. One is that in certain fields (some musical instruments, sports, and art media, particularly, as well as anything requiring muscle memory for proper mastery) a certain amount of hands-on instruction frequently helps new students avoid developing bad habits that will hinder their later development. Another is that after a certain point, it's often difficult or impossible to proceed further in a study without formal education, partly because of the benefits of conducting research in a community setting, partly because the works necessary to advance in the subject simply aren't available or reasonably priced for the general public. In many if not most fields, it's simply not possible to do PhD-level work without producing highly specialized work that passes muster with the experts in the field, and striking out on your own (the "independent scientist" gambit) is almost always counterproductive.

The sourcing problem
Another problem with autodidacticism is quality control. If you step into a new field, you aren't always going to know what the best introductory materials are, and you may be very inclined to select only those books that support your personal beliefs and ideologies. People with extremist political axes to grind know this; Regnery Publishing's Politically Incorrect Guides, for example, are formatted to look similar to better-known, more politically neutral series like Wiley's ...for Dummies books, partly so that their target audience doesn't feel a need to look outside the walls for information, partly to fool people looking for information in general. And some subjects (such as ecology and some areas of history, like the American Civil War and the history of Israel) are so politically charged that picking through the good and the bad actually requires a somewhat advanced grasp of basic science or history in order to understand what's sensible or not.

Why bother then?
Plenty of reasons. Some amount of self-education is necessary to avoid looking like a fool in a debate, for example. An understanding of basic astronomy and astrophysics is important to writing workable science fiction; history and parliamentary procedure make for a much better understanding of politics. And it never hurts to know a second, or third, or fourth language, or to be able to do basic first aid or car repairs.

This article isn't meant to discourage you by any means from learning whatever you want in whatever fashion you find easiest. It's just that you do ultimately have to find ways of avoiding pitfalls like bad information, confirmation bias, and the Dunning-Kruger effect. Any form of inquiry and/or study requires a certain amount of humility to allow you to backtrack and correct when you've gone off the trail; self-teaching puts the onus on you to make sure you're not being fooled or, worse, fooling yourself. That in mind, go forth and learn.

Famous

 * Frederick Douglass
 * Thomas Edison
 * Oliver Heaviside
 * Abraham Lincoln, apprenticed lawyer
 * Joni Mitchell, who could never manage playing a standard guitar and invented her own tunings to compensate
 * Srinivasa Ramanujan, mathematician
 * Orville and Wilbur Wright

Infamous

 * Eliezer Yudkowsky
 * Cesar Millan
 * Adolf Hitler
 * Most pundits in the US (ie. Glenn Beck)

Resources for the would-be autodidact

 * The Open Courseware Consortium, the CK-12 Foundation, Coursera, and Apple iTunes U are among a great many sources for freely available texts and courseware from kindergarten through university level work.
 * ...for Dummies and Teach Yourself are both well-established brands for English-language self-instruction on everything from computers to games to crafts to languages. There are others, but those are probably the two most important. (Klutz Press, Usborne and Dorling-Kindersley provide similar titles for children and young adults.)