Steve Jobs has changed the course of personal computing during his two stints as chief executive of Apple, while his idiosyncratic style and the iconic iPod, iPhone, iPad and Mac have made him a household name.

Deemed irreplaceable by many Apple fans and investors, pancreatic cancer survivor Jobs said on Monday he would take a medical leave of absence to concentrate on his health -- two years after a previous health-related absence.

The 55-year-old Apple co-founder said nothing about the nature of his latest medical concerns despite widespread criticism over the inadequacy of past disclosures about his health.

During his past illnesses, showman Jobs continued to unveil the latest Apple products at glitzy company events, leaving customers, employees and analysts alike to speculate on his gaunt appearance.

Jobs revived Apple's fortunes after a 12-year absence in large part due to a passion for design that produced the phenomenally popular iPod.

More than a quarter of a billion of the digital music players have been sold since the first one was released in 2001.

In 2007, Apple transformed the cellphone market with the touchscreen iPhone, dubbed the Jesus phone for its quasi-religious following, and a year ago Apple created a whole new market for tablet computers with the launch of the iPad.

Steve Jobs is seen by the market to be a major force in Apple's strategic direction, Richard Windsor, global technology specialist at Nomura, said on Monday.

In recent years, however, questions about his health and his ability to lead the company he created have threatened to overshadow his past achievements and cloud his prospects.

After recovering from a rare and highly treatable form of pancreatic cancer in 2004, Jobs took medical leave for the first half of 2009, saying his health issues were more complex than a hormonal imbalance he had previously complained of.

Rumors abounded on blogs about whether he was suffering from complications related to his previous tumor. He later said he had a liver transplant during this period.

Now, some analysts are betting that Jobs could even be getting ready to step down for good and hand Apple over to an unknown successor -- possibly Chief Operating Officer Tim Cook, who has run the company before in Jobs's absence.

CHANGING THE WORLD

Jobs, a college dropout, Buddhist and son of adoptive parents, started Apple Computer with his friend Steve Wozniak in the Jobs family garage in Silicon Valley in the late 1970s.

The company soon introduced the Apple 1 computer. But it was the Apple II that became a huge success and gave Apple its position as a critical player in the then-nascent PC industry, resulting in a 1980 IPO that made Jobs a multi-millionaire.

In 1983, Jobs famously lured John Sculley, then CEO of Pepsi, to lead Apple with the question: Do you want to spend the rest of your life selling sugared water or do you want a chance to change the world?

A year later came the Macintosh, the world's first commercially successful computer with an easy-to-use graphical user interface.

Despite the success of the Mac, or perhaps partly because of it, the relationship between Jobs and Sculley soured, and in 1985 the board removed most of Jobs's powers and he left the company, selling all but one share of his Apple holdings.

Apple's purchase of NeXT -- the computer company Jobs founded after leaving Apple -- in 1997 brought Jobs back to Apple. Later that year he became interim CEO and in 2000 the company dropped interim from his title.

In 2001, Apple launched the iPod, whose elegant and simple design cemented Jobs' legacy as an innovator able to marry technology and media.

In addition to his Apple pursuits, Jobs co-founded animation studio Pixar in 1986 with Edwin Catmull and Alvy Ray Smith, buying Lucasfilm's computer graphics division for $10 million.

In 1995 came Toy Story, the breakout computer animated film. Academy Award winners including The Incredibles, Ratatouille and Monsters, Inc. have followed.

Jobs became a board member and the largest individual shareholder of Walt Disney Co when Disney purchased Pixar, now known as Disney-Pixar, in 2006.

For a timeline of key dates in Jobs's health problems, double click on

For an instant view on Jobs's medical leave, double click on

(Additional reporting by Nichola Groom and Bernie Woodall; Editing by David Cowell)

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