The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday 24 June 2009

The article below on the chief executive of Apple referred to memorable TV ads aired in 1984 to promote the Apple Macintosh computer and said these portrayed "mindless conformity in Jobs's supreme rival, Bill Gates of Microsoft". The ads were actually tilting at the industry's giant of the day, IBM. A young Bill Gates is on record praising the Mac.

When Steve Jobs rolled out the Apple Macintosh computer in January 1984 he did so, true to form, by commissioning one of the most memorable TV adverts ever created. It bore several classic Jobs hallmarks. It was cool. It was darkly funny. And it seemed to portray a mindless conformity in Jobs's supreme rival, Bill Gates of Microsoft.

Modelled on the George Orwell novel Nineteen Eighty-Four, it showed row upon row of grey-suited, grey-faced male automatons (for which read PC users) sitting in front of a huge screen upon which Big Brother pontificates. A blonde woman in white and red running gear - Ms Apple by implication - hurtles towards him and hurls a mallet at the screen, which erupts into a fireball just as Big Brother is saying, "We shall prevail!"

Close observers of the ever more mysterious world of Steve Jobs will have remembered that advert this weekend with a wry chuckle. For, in the last few days, the behaviour of Apple's chief executive and his top team has become increasingly erratic, bordering on bizarre, with definite shades of Big Brother.

Over the weekend we learned that Jobs had had a liver transplant two months ago. You could almost hear the sharp intake of breath among geeks everywhere, particularly in the west-coast techno-Mecca of Silicon Valley. At last there was an explanation for his gaunt appearance and evident weight loss, for his disappearances and for this year's six-month break from work.

But consider the manner in which the news came out. It broke late on Friday night - that witching hour beloved of buriers of bad news - at the end of a day that had also seen Apple launch its latest iPhone. The Wall Street Journal had the exclusive and, unusually for them, ascribed it to no source at all, provoking a mass of blogger speculation that the source could only have been Jobs himself. All that Apple would say on the subject was to repeat parrot-like the phrase that "Steve continues to look forward to returning at the end of June". Big Brother would have been proud.

The news of the liver transplant was just the latest of a series of events that saw Jobs and Apple dig themselves deeper and deeper into a pit of secrecy surrounding his health. The first the world knew about his problems was in August 2004, when it was announced he had been treated for a rare form of pancreatic cancer.

Later it dribbled out that in fact the diagnosis had been made months earlier, in October 2003, and that he had tried initially to beat it with a peculiar dietary treatment (we're talking about a Buddhist vegetarian approach here).

Earlier this year came the baffling claim that he had a relatively simply treated "hormone imbalance", followed just days later by the announcement that Jobs would be stepping down for six months to focus on his health.

The relative news blackout over almost five years has prompted desperate attempts by Jobs's devoted band of followers to reach the truth. Bloggers have tried to confirm that April's liver transplant took place in Memphis, Tennessee, by tracking the movements of Jobs's private plane. They have also pored over records of property transactions near the Memphis hospital that specialises in such treatments, to identify the mansion house they believe he bought while preparing for and recovering from the operation.

Why go to such extraordinary lengths to find out information that Jobs clearly prefers to keep private? Why does his health matter so much to anyone other than himself and his close family and friends?

Here's a clue to the answer to those questions: Apple share prices. When Jobs took over the company for the second time in 1996 its stock was trading at about $4 a share; now it approaches $140.

Or look at more micro movements. When his six-month break was announced in January, share values plummeted to as low as $78.20. Even unfounded rumours about his health can move the markets: last December, Apple stock went up simply on unconfirmed reports that Jobs had been spotted looking well at his favourite frozen yoghurt outlet in Palo Alto. On another occasion, an inaccurate blog report that he had suffered a heart attack depressed shares by more than 5%.

Such figures, impressive though they are, are mere symptoms of the peculiar power of Steve Jobs; they do not provide a diagnosis. For that one has to look back into the cluttered garage of Jobs's family home in California where, in 1976, aged just 21, he joined forces with Steve Wozniak to create their first personal computer. They built 25 of the first Apple I prototype, with the help of an order from a local company, and within just four years had exploded from a garage industry into a major international technology giant.

Millions of words and a library full of books have been dedicated to trying to understand how Jobs did it. But two characteristics stand out beyond all others. First, Jobs is a man who does not suffer from self-doubt: everything he does is imbued with absolute self-confidence.

Second, he is obsessive about Apple and its products, which he engages with at a level of detail virtually unheard of among his fellow CEOs. Rob Enderle, a technology analyst who has been covering Apple for more than two decades, says that Jobs has "redesigned Apple around him. In any product, he's involved from its first design stage to its final presentation."

That attention to detail, Enderle says, includes coal-face negotiations with phone carriers and suppliers in which Jobs uses his legendary temper to scare other companies into submission. Enderle tells the story of one firm that supplied a faulty computer chip to Apple. Though the fault was Apple's own, Jobs apparently insisted the supplier rebuilt the chip in an impossible 60 days, threatening never to work with them again if the deadline was missed.

"That's the kind of thing that scares people to death. They know that if you let him down you can lose your contract, your job or even your company," Enderle says.

Jobs has a similar nerve-jangling reputation among Apple's 35,000 employees. Workers at the company's Cupertino headquarters have a phrase - "being Steved", meaning to be at the receiving end of his wrath. "He uses fear more effectively than any CEO I've ever seen," Enderle says.

Leander Kahney runs an Apple analysis website, Cultofmac.com, and is author of Inside Steve's Brain, which seeks to understand his creative genius. He believes that since Jobs returned to Apple in 1996, having resigned from his own creation 11 years previously, he has set out to remould it in his own image.

The new Apple bears the imprint of Jobs's obsessiveness. "When they take on a problem at Apple they discuss it from every possible angle until they have absolutely mastered it."

Kahney points to a telling - and rare - anecdote from Jobs's own private life. A few years ago he needed to replace the family washer-dryer. Where most people might go online and select a model that looked right for them, Jobs embarked on a two-week consultation process with his wife and children. Every night around the dinner table they explored a different aspect of the ideal machine - how much water it used, its ecological footprint, its cleaning ability and how it impacted on clothes. Then he went out and bought one of the most expensive German models.

Apple is sticking to that Big Brother mantra that Jobs will return later this month, though it adds that he may come back initially in a part-time capacity. The question, though, is can the culture that he has created in his own image survive him?

Other senior executives have been stepping up to the plate since Jobs's enforced absence. Tim Cook is widely tipped as a possible replacement as CEO. He has run the show for the last few months, proving himself to be a diligent and safe pair of hands, but analysts wonder whether he lacks the creative spark that Jobs has had in abundance. Last Friday's launch of the latest iPhone was competent, but where was the magic?

The same issue exists in reverse for Jonathan Ive, Apple's legendary designer. As a design artist he is unsurpassed, but he lacks the clout and the fear factor that Jobs uses to generate loyalty and drive results.

And then there's that other overwhelming question, which has little bearing on Apple's commercial future but has every bearing on Jobs's personal prospects. Why would someone facing the threat of cancer spreading from his pancreas to other organs want to bother with mere gadgetry any longer? Hasn't he got bigger worries now on his mind than whizzbang phones?

There is another clue here provided by a speech he gave in 2005 - just after his surgery for cancer - to a class of graduating students at Stanford. He spent a third of the speech talking directly about death. "Remembering I'll be dead soon is the most important tool I've ever encountered to help me make the big choices in life. Remembering that you are going to die is the best way I know to avoid the trap of thinking you have something to lose. You are already naked. There is no reason not to follow your heart."

In a few weeks' time we will know - Big Brother permitting - the choice that Jobs has made this time round. Given his record, it is a safe bet that he will yet again follow his heart. And that his heart will be Apple.

Supreme leaders: Other bosses who define their companies

Richard Branson/Virgin

Richard Branson is less businessman than publicity-seeking missile. His stunts (the boat racing, the balloon flights) may be a way of getting cheap advertising for the Virgin group, but they also express a show-off streak. He appears on TV, he blogs, he tweets - and in his spare time he starts up new businesses. Virgin has begun more than 200 companies, doing everything from planes and trains to selling wine and offering flights into space. Binding all these ventures together is nothing stronger than the company logo, and the always-smiling, ever-bearded Branson.

Oprah Winfrey (various)

Of all the big egos on this list, Winfrey is surely the only one to have had an academic course created after her, at the University of Illinois: History 298 - Oprah Winfrey, the Tycoon. Winfrey used the knockout ratings for her talk show to set up Harpo, her own production company. She then began O, The Oprah Magazine - perhaps the most successful startup in the history of the magazine industry - and went into making TV films. Winfrey claims to believe in management by instinct, and is so wary of investing in the share market that she once hoarded $50m in cash.

Mukesh Ambani/Reliance

Mukesh Ambani is probably the wealthiest billionaire you've never heard of. He runs India's largest private business, the Reliance conglomerate, which deals in everything from natural gas to Bollywood movies. Mukesh and his younger brother Anil inherited the Reliance conglomerate from their father, but have since fallen out. Easily the highest-profile businessman in India, Ambani is building in Mumbai what is reputed to be the priciest home in the world: 60 storeys high and worth $1bn.

Aditya Chakrabortty