Hard to understand, difficult to work with and deemed irreplaceable by many Apple fans and investors, Steve Jobs has made a life defying conventions and expectations.

And despite years of signs of poor health, his resignation as chief executive of Apple Inc caused a global gasp as the world contemplated the future of an icon and the company he symbolizes.

Steve Jobs is the most successful CEO in the U.S. of the last 25 years, said Google Inc Chairman Eric Schmidt, who used to sit on Apple's board but stepped down because of overlapping business interests.

He uniquely combined an artist's touch and an engineer's vision to build an extraordinary company, one of the greatest American leaders in history, Schmidt said in a statement.

A college dropout, Jobs floated through India in search of spiritual guidance prior to founding Apple -- a name he suggested to his friend and co-founder Steve Wozniak after a visit to a commune in Oregon he referred to as an apple orchard.

With his passion for minimalist design and marketing genius, Jobs changed the course of personal computing during two stints at Apple and transformed the mobile market.

The iconic iPod, the iPhone -- dubbed the Jesus phone for its quasi-religious following -- and the iPad are the creation of a man known for his near-obsessive control of the product development process.

Most mere mortals cannot understand a person like Steve Jobs, Guy Kawasaki, a former Apple employee who considers Jobs the greatest CEO in the history of man, said recently. He's just got a different operating system.

Charismatic, visionary, ruthless, perfectionist, dictator - these are some of the words that people use to describe the larger-than-life figure of Jobs, who may be the biggest dreamer the technology world has ever known, but also a hard-edged businessman and negotiator through and through.

Steve Jobs is the business genius of our generation, former eBay Inc chief Meg Whitman said recently. His contributions to Apple, his contributions to technology, frankly his contributions to America, are unparalleled in the business world. He is amazing.

Former nemesis Bill Gates, the co-founder of Microsoft, has called Jobs the most inspiring person in the tech industry and President Barack Obama has held him up as the embodiment of the American Dream

It's hard to imagine a bigger success story than Steve Jobs, but rejection, failure and bad fate have been part and parcel of who he is. Jobs was given away at birth, driven out of Apple in the mid-80s and struck with cancer when he finally had regained the top of the mountain. His resignation as CEO on Wednesday comes at the relatively young age of 55.

I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know. Unfortunately, that day has come, he said in a brief letter announcing his resignation.

A source close to Jobs said he plans to be active in his new role as chairman of Apple's board.

Jobs grew up with an adopted family in Silicon Valley, which was turning from orchards to homes for workers at Lockheed and other defense and technology companies.

Electronics friend Bill Fernandez introduced him to boy engineer Wozniak, and the two Steves began a friendship that eventually bred Apple Computer.

Woz is a brilliant engineer, but he is not really an entrepreneur, and that's where Jobs came in, remembers Fernandez, who was the first employee at Apple.

Wozniak said that his goal was only to design hardware and he had no interest in running Apple.

Steve Jobs' role was defined -- you've got to learn to be an executive in every division of the company so you can be the world's most important person some day. That was his goal, recently joked Woz, who is still listed as an employee reporting directly to Jobs, even though he has not worked at Apple for years.

AWFUL-TASTING MEDICINE

Jobs created Apple twice -- once when he founded it and the second time after a return credited with saving the company, which now vies with Exxon Mobil as the most valuable publicly traded corporation in the United States.

Every day to him is a new adventure in the company, said Jay Elliot, a former senior vice president at Apple who worked very closely with Jobs in the eighties. He is almost like a child when it comes to his inquisitiveness. Steve has such a thirst of understanding for what's going on in the company. What he is intolerant about it - politics, bureaucracy.

But the inspiring Jobs came with a lot of hard edges, oftentimes alienating colleagues and early investors with his my-way-or-the-highway dictums and plans that were generally ahead of their time.

Elliot was a witness to the acrimony between Jobs and former Apple Chief Executive John Sculley who often clashed on ideas, products and the direction of the company.

The dispute came to a head at Apple's first major sales meeting in Hawaii in 1985 where the two just blew up against each other, Elliot said.

Jobs left soon after, saying he was fired.

It was awful-tasting medicine, but I guess the patient needed it. Sometimes life's gonna hit you in the head with a brick. Don't lose faith, he told a Stanford graduating class in 2005. He returned to Apple about a decade after he left, working as a consultant. Soon he was running it, in what has been called Jobs' second act.

To this point, he has reinvented the technology world four or five times, first with the Apple II, a beautiful personal computer in the 1970s; then in the 1980s with the Macintosh, driven by a mouse and presenting a clean screen that made computing inviting; the ubiquitous iPod debuted in 2001, the iPhone in 2007 and in 2010 the iPad, which a year after it was introduced outsold Macs.

LESS IS MORE

How did he do it? Design fans, Apple employees and Jobs acquaintances credit a natural design sense driven to simplify. Jobs' return to Apple was a study in reduction.

Ed Niehaus, who was wooed and hired by Jobs to do PR for resurgent Apple, remembers an elevator ride that everyone in Silicon Valley has heard of, but seemed more myth than reality. It was soon after Jobs' triumphant return and he was axing product plans -- and people.

Niehaus recalled: I once rode down an elevator, not that many floors. We got in the elevator and the next floor a young woman got in, and I could see her go, 'oops, wrong elevator.' And Steve said, 'Hi, who are you?' and introduces himself to her -- 'I'm Steve Jobs' and turned on the charm and said, 'What do you do?' and all this sort of thing. And the door of the elevator opens at the bottom, and he says, 'We are not going to need you.' And we walk away.

Apple was bloated, Niehaus added, and Jobs was bringing back simplicity and focus.

What makes Steve's methodology different from everyone else's is that he always believed the most important decisions you make are not the things you do - but the things that you decide not to do. He's a minimalist, former CEO Sculley -- who was recruited by Jobs, watched him build the Mac, and then helped throw out the Apple founder in a boardroom battle -- told the CultofMac news website in 2010.

A few steps to Apple design have leaked out over the years, despite the obsessive secrecy that is part of the company culture. An Apple engineer outlined a long development process at a conference blogged by Businessweek in 2008.

A new product or feature begins with 10 ideas -- good ideas, no also-rans, which are presented as pixel-perfect mockups. Apple culls the 10 to three, which are tried out for months more, before a final star is chosen.

Meanwhile, the design team meets for two types of weekly meetings -- one to brainstorm with no limits, and one to focus on getting the product out the door, BusinessWeek described.

When Steve Jobs weighs in, it is with a simple set of verdicts: insanely great; really, really great; and shit, Niehaus recalled.

Basically Steve tells you exactly what he wants and you just go build it, said one former iPhone engineer, who declined to give his name.

He remembers working on one project for two months. Steve said 'What is this shit? Why are you wasting my time?' he recalled.

Being chewed up and spat out by Jobs is an experience most Apple employees who have come in contact with Jobs can relate to. And Jobs is known to like people who can stand up to him.

I never asked you to start, so why should I ask you to stop? Jobs told another former Apple employee, who wanted to know whether he should continue to work on a project that was being questioned by the forceful CEO.

Jobs likes to push. From the very start, people told tales of him putting his -- often dirty -- feet on the table in meetings. Others tell of Jobs putting down their company, making them defend themselves in interviews.

He was clearly looking for someone who could stand up to him, said another former member of the top team. He remembers Jobs and Tim Cook, who is taking over as CEO, as the metronome of the company, with vastly different personal styles and exactly the same insane attention to detail.

Jobs, in fact, revels in details, many a time irking everyone around him with his obsessiveness.

Apple's first CEO Michael Scott has said that Jobs spent weeks contemplating how rounded the edges of the Apple II case should be.

He put white earbuds in the ears of everyone on the planet, and shut us all in to our own little pods of experience, said Niehaus, who is in awe of Jobs' taste and talent. Jobs, given a Gulfstream jet by his appreciative board, probably hasn't flown commercial in years, and everyone who sits down with an iPod next to someone they don't want to acknowledge gets a little bit of that experience.

He understands envy as well as anybody on the planet, and he carries it around with him, triple parking his car, because he can. Part of what he is selling is envy, Niehaus said.

THE REAL STEVE JOBS

Even Jobs' appearance simplified over the years. When he returned to Apple after his decade away, he wore fancy white shirts and vests and even a pin stripe suit to introduce new products.

The black mock turtleneck and jeans that have become the defining Jobs outfit showed up at more comfortable settings, when Jobs wooed developers, in the late 1990s. But he pulled the iPod out of a jeans pocket to introduce the music player in 2001. From then on, he's barely taken off the outfit.

The jeans and running shoes flashed under his academic gown when he gave the Stanford commencement speech in 2005, and he's wearing a black mock turtleneck sitting next to President Barack Obama at a 2011 dinner with Silicon Valley titans. On Obama's other side was Facebook founder Mark Zuckerberg, who wore a jacket to the event.

Jobs himself describes his world as very simple.

For the past 33 years I have looked in the mirror every morning and asked myself, 'if today were the last day of my life, would I want to do what I am about to do today. And whenever the answer has been 'no' for too many days in a row, I know I need to change something, he told Stanford University students in the soul-baring commencement address.

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, he said.

That kind of earnest, almost naive hope, combined with ruthless dismissal of whatever misses his lofty standards, are a potent mix for those around him.

His approval is an addictive drug, said Niehaus. I think that most people would knock themselves out to have that experience again, once they've had it. It's that defining. It is a really tremendous experience.

APPLE 3.0

Jobs has been on leave three times since 2004, and he clearly has thought about an Apple without him. Jobs has had a liver transplant and a rare form of pancreatic cancer.

For years every presentation by Jobs sparked discussions of whether the gaunt executive looked better or worse.

Jobs has talked about his own mortality, and said it has been a major driver in his life and work.

Remembering that I'll be dead soon is the most important tool I've ever encountered to help me make the big choices in life, Jobs said in the commencement speech at Stanford in 2005. Because almost everything -- all external expectations, all pride, all fear of embarrassment or failure -- these things just fall away in the face of death, leaving only what is truly important.

Jobs and the board had a succession plan -- put Cook in charge -- and has a well-respected team. Jobs has put extraordinary effort into finding people who he says are 10, 20, 50 times better than average, he told Time magazine, adding that there were no prima donnas when great people got together.

He has a close circle of advisers he relies on. Having a close circle of people was really important to him, Elliot said.

Many Apple watchers and investors say that the company has a deep bench, led by Cook. But for others, that just doesn't ring true.

The former engineer whose months of works was dismissed by Jobs with a single curse doesn't see much strength in the ranks. Steve is the visionary, he said. If something happens, it's like 'Let's ask Steve'.

(Editing by Tiffany Wu and Ted Kerr)

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