My, hasn't it grown? World of Warcraft celebrates its fifth birthday this month, and the population of Azeroth – the virtual world where this online game takes place – is around 11.5 million. More people now play World of Warcraft, or WoW, than live in Greece.

WoW is a sword'n'sorcery game in which players control virtual characters who whizz about killing monsters, collecting treasure, fighting or ganging up with one another. Azeroth has a functioning virtual economy, and (mindbogglingly) an exchange rate with the real world: a WoW gold piece, enough to buy you some virtual barbecued boar ribs, is currently worth about one and a half euro cents. Also, WoW has some wicked magic swords. Yet grown-ups play it. In fact, seduced by the beauty of this whole new world that's theirs to explore, they find it eats their lives.

WoW is such a colossal and pernicious time-suck that it can have you slaying goblin pirates till 4am. It is also, undoubtedly, a work of art. But what sort of work of art? I have a theory. Computer games are often described in terms of what they resemble. They appear on screen; they feature dwarves, giant spiders and gurgling fish-men; they have cinematic trailers and blockbuster-style launches – so they must be like films, right? Wrong.

Games aren't trying to be films any more than songs are trying to be poems, paintings are trying to be photographs, or Ginsters pasties are trying to be food. When it comes to WoW, the comparison is especially skewed. Films, with the exception of Andy Warhol's, are all about narrative. They tell stories. But narrative is probably WoW's weakest suit. Here's a typical sequence: kill demon boar, kill another demon boar, get ambushed by ghoul, swear, wake up in graveyard, traipse in ectoplasmic form across open country (swearing all the way) to your own dead body, resurrect self, get ambushed by ghoul again, awake in graveyard again, swear again.

Nothing is permanently at risk. Good and evil are simply team colours, not moral convictions. Endlessly repeatable, the "quests" are more like errands or even rituals. Your character is changed by these quests only by becoming richer, more powerful and better dressed. Azeroth is the only place where I can get away with wearing gem-encrusted black leather trousers.

So here's my theory: WoW doesn't resemble a film. It resembles, rather, a medieval cathedral. And a magnificent one: it is the Chartres of the video-game world. Like a cathedral, it is a supreme work of art that is, on a brick-by-brick basis, the creation of hundreds of artisans and craftsmen, many of whom will be long gone by the time it comes to completion; indeed, since WoW is in a state of permanent expansion, it may not ever be "complete". All those programmers are the modern-day equivalent of stonemasons, foundation-diggers and structural engineers.

Cathedrals don't really have narratives either, but they do have a mythos – a system of stories – behind them. And oh boy, does WoW have a mythos: just Google Kil'Jaeden and the Shadow Pact or Kel'Thuzad and the Forming of the Scourge, and you'll wish you never had. The Azeroth has a historical back-story involving aeons of strife and sacrifice, unspeakable cataclysms, mighty heroes, and tonnes and tonnes of that sort of guff.

But the mythos is there in the background: it's part of the furniture, rather than part of the action – in the same way that windows of stained-glass martyrs, or narrative frescoes of the Passion, serve as a backdrop to ritual observances in a cathedral. And the observances of WoW are, like those that take place in a cathedral, calmingly repetitious and governed by rules. Instead of a series of Hail Marys, you're handed a target number of flowers to collect, or giants to kill; instead of manna from heaven, you are rewarded with experience points in the XP bar.

It is rich in decorative detail, but the decorative detail is not the point. Azeroth's architecture is a glorious space for glorious things to happen in. And, like a cathedral, it is above all a social space, for communal experience. That's what has given it its longevity. Five years isn't long in terms of the life of a cathedral; for a computer game, it's an eternity, given that you can finish most in a matter of hours. The people who stay in WoW join guilds, make friends online, go questing in groups and spend hours (with only a bit of giant-slaying) talking in the chat channels. It's as much a social networking site as a videogame. You log on and gossip in its pews.

Gone are the days when families gathered round the wireless. Inconceivable, now, would be the simultaneous nationwide toilet flush during the break of the final MASH episode. Art is increasingly consumed in isolation – through earplugs, on the computer, on demand. Yet there's still a thirst for a communal experience of culture. Hence the boom in live music, and in interactive, watch-on-the-night shows like The X Factor. And then there's World of Warcraft, perhaps the daddy of them all: a cathedral without a god, where you and your gang can hang out – dressed in leather, killing goblins and eating ribs.