Wikipedia is the most widely used reference work in the world. That statement is both ordinary and astonishing: it's a simple reflection of its enormous readership; and yet, by any traditional view about how the world works, Wikipedia shouldn't even exist, much less have succeeded so dramatically in the space of a single decade.

The cumulative effort of Wikipedia's millions of contributors means you are a click away from figuring out what a myocardial infarction is, or the cause of the Agacher Strip war, or who Spangles Muldoon was. This is an unplanned miracle, like "the market" deciding how much bread goes in the store. Wikipedia, though, is even odder than the market: not only is all that material contributed for free, it is available to you free; even the servers and system administrators are funded through donations. That it would become such a miracle was not obvious at its inception and so, on the occasion of its 10th birthday, it's worth retelling the improbable story of its genesis.

Ten years ago today, Jimmy Wales and Larry Sanger were stuck trying to create Nupedia, an online encyclopedia with a seven-step publishing process. Unfortunately, that also meant seven places where things could grind to a halt. However, after nearly a year of work, almost no articles had actually been published.

So, 10 years ago tomorrow, Wales and Sanger decided to try a wiki, as a way of cutting through some of that process. Sanger sent an email to Nupedia collaborators about this new way of working, saying: "Humour me. Go there and add a little article. It will take all of five or ten minutes."

The "Humour me" bit was necessary because the wiki is social media at its most radical. Invented in the mid-90s by Ward Cunningham, a wiki has at its core only one technical function: edit. You don't need permission to add, alter, or delete text, and when you are done, you don't need permission to publish.

More remarkably, though, a wiki also has at its core only one social operation: I care. The people who edit pages are the ones who care enough to edit them. Putting the people who care in charge, rather than anointing experts or authorities, was so radical that Wales and Sanger didn't propose replacing Nupedia with a wiki. Instead, they proposed using the wiki to generate raw material for Nupedia.

The participants, however, had other ideas. The ability to create an article in five minutes, and to make an existing article a little better in less, was so infectious that in a matter of days there were more articles on the nascent wiki than on Nupedia. The wiki was so good, and so different from Nupedia, it was soon moved to its own site. Wikipedia was born. (Nupedia was shut down a few months later; Sanger also left the project.)

That process continues today, making Wikipedia an ordinary miracle for more than 250 million people a month. Every single day for the last 10 years Wikipedia has got better because someone – several million someones in all – decided to make it better. Sometimes that meant starting a new article. Mostly it meant editing an existing one. Occasionally it meant defending Wikipedia against vandalism. Always it meant caring. Most participants care a little, editing only one article. A handful care a lot, contributing hundreds of thousands of edits, across thousands of articles, over years. Most importantly, taken together, all of us have contributed enough to make Wikipedia what we have today. What looks like a stable thing is in fact a result of ceaseless attempts to preserve what is good, and to improve what isn't. Wikipedia is best understood not as a product with an organisation behind it, but as an activity that happens to leave an encyclopedia in its wake.

That shift, from product to activity, has involved the most amazing expansion of peer review ever: Wikipedia's editor-in-chief is a rotating quorum of whoever is paying attention. Many of Wikipedia's critics have focused on the fact that the software lets anyone edit anything; what they miss is that the social constraints of the committed editors keep that capability in check. As easy as the software makes it to do damage, it makes it even easier to undo damage.

Imagine a wall where it was easier to remove graffiti than add it: the amount of graffiti on such a wall would depend on the commitment of its defenders. So with Wikipedia; if all its passionate participants were to stop caring, the whole thing would be gone by next Thursday, overrun by vandals and spammers. If you can see Wikipedia right now, it means that again, today, the good guys won.

Wikipedia isn't perfect, of course. Many mediocre articles need improvement. The editors are not diverse enough in age, gender or ethnicity. Biographies of the living remain a persistent site of mischief. Defences erected against vandals and spammers also see off novices and exhaust old-timers. But Wikipedia isn't just an activity at the level of the articles; from the individual edits all the way up to the culture of the whole, Wikipedia is a public good created by the public, so it falls to the people who care to try to take on these problems as well. As long as that culture continues to embrace "be bold" as a core value, its status as one of the largest cumulative acts of generosity in history will persist. So happy 10th birthday to Wikipedia, and ardent thanks to the millions of people who have added and altered and argued and amended, the people who have created the most widely used reference work in the world. Thanks for telling us the story of the Stonewall riots and how Pluto got demoted to "dwarf planet"; about the Great Rift Valley and the Indian Ocean tsunami; about lion fish and tiger teams and bear markets. And along with the birthday wishes, here's hoping enough of us keep caring enough to be able to greet you again, in rude good health, for your 20th.