Steve Jobs, a co-founder of Apple Inc., who introduced simple, well-designed computers for people who were more interested in what technology could do than in how it was done, died Wednesday at age 56.

In a brief statement, Apple announced the death but did not say where he died. Mr. Jobs, who suffered from a rare form of pancreatic cancer and had a liver transplant in 2009, stepped down as Apple’s chief executive on Aug. 24.

An original thinker who helped create the Macintosh, one of the world’s most influential computers, Mr. Jobs also reinvented the portable music player with the iPod, launched the first successful legal method of selling music online with iTunes and reordered the cellphone market with the iPhone. The introduction of the iPad also jump-started the electronic-tablet market, and it now dominates the field.

Calculating that people would be willing to pay a premium price for products that signaled creativity, Mr. Jobs had a genius for understanding the needs of consumers before they did.

He knew best of all how to market: “Mac or PC?” became one of the defining questions of the late 20th century, and although Apple sold a mere 5 percent of all computers during that era, Mac users became rabid partisans.

Mr. Jobs was the first crossover technology star, turning Silicon Valley renown into Main Street recognition and paving the way for the rise of the nerds, such as Yahoo founders Jerry Yang and David Filo, and Google founders Larry Page and Sergey Brin.

And by changing the way people interacted with technology, Jobs and Microsoft founder Bill Gates transformed their era in much the same way Henry Ford and John D. Rockefeller revolutionized theirs with the mass-produced automobile and the creation of Standard Oil.

As a 21-year-old college dropout in 1976, Mr. Jobs founded Apple with his friend Steve Wozniak. He led the company to multimillion-dollar success within five years but was forced out by the time he was 30.

Mr. Jobs started another computer firm, Next, whose technology was used to create the World Wide Web. He later took over a foundering computer animation company and turned it into the Academy Award-winning Pixar, maker of “Toy Story” and “Finding Nemo.”

Returning to Apple in his 40s, Mr. Jobs restored the company to profitability by paring down the product line and being a leader in innovation.

Known for his complex and combative temperament, Mr. Jobs was a private man. But in a June 2005 commencement address at Stanford University, he talked openly about his pancreatic cancer, diagnosed in 2004, in a video that became an Internet sensation. He later became furious about speculation over his health in mid-2008, when he appeared in public looking gaunt. Later that year, he took a leave of absence from the company to have a liver transplant.

In January, he took another medical leave. On Aug. 24 he stepped down as Apple’s chief executive, becoming chairman of the board. Apple’s share price immediately dropped 5 percent on the news, but it rebounded the next day. “Steve Jobs running the company from jail would be better for the stock price than Steve Jobs not being CEO,” one stock analyst told Fortune magazine in early 2011.

His innovations led Business 2.0 to call him “easily the greatest marketer since P.T. Barnum.” One of his employees, noting that Mr. Jobs was able to persuade people to believe almost anything, coined the phrase “reality distortion field” to describe his ability to warp an audience’s sense of proportion. Mr. Jobs described the Macintosh computer, for example, as “insanely great.”

Maybe it was. It was designed for the home and creative professional user, not the computer-science student or the bottom-line-oriented businessman. During a famous 1979 visit to Xerox Parc, the hotbed of innovation where the computer mouse, networking and graphical user interface were invented, Mr. Jobs and Wozniak learned that computer users did not have to type in a series of commands to make the computer perform; they could simply point their mouse at a picture of a file and click it to get the file to open.

That recognition sparked a flurry of innovation unmatched in technology until the designers of Microsoft’s operating software mimicked the look and feel of their competitors with Windows 95.

Years later, discussing computer design in another context, Mr. Jobs said: “Most people make the mistake of thinking design is what it looks like. People think it’s this veneer, that the designers are handed this box and told, ‘Make it look good!’ That’s not what we think design is. It’s not just what it looks like and feels like. Design is how it works.”

He could control how it works because Apple “makes the whole widget,” as Mr. Jobs repeatedly said — software and hardware. The company introduced monitors with color screens long before others. Locked out of many retail chains because of its small market share, Apple responded with its own distinctively branded stores, to which users flock like pilgrims. The Mac, Mr. Jobs saw, could become the hub of a digital lifestyle.

Not everything worked out. A 1983 computer, Lisa, failed miserably. Even the “insanely great” Macintosh, sold without a letter-quality printer and incompatible with other computers, had a difficult start, even though it launched the desktop publishing revolution. But that wasn’t the first rough start in Mr. Jobs’s life.

Steven Paul Jobs was born Feb. 24, 1955, in San Francisco to unwed parents, University of Wisconsin graduate student Joanne Carole Schieble and Syrian exchange student Abdulfattah Jandali. Paul and Clara Jobs adopted him shortly after his birth.

Mr. Jobs grew up in Northern California and showed an early interest in electronics. While working on a project as an eighth-grader, he saw he was missing a part. Undaunted, he called William Hewlett, co-founder and president of Hewlett-Packard, who prepared a bag of parts for him — and offered him a summer internship.

Mr. Jobs attended Reed College in Portland, Ore., in 1972. He dropped out after a semester because of financial issues but continued to audit classes at the liberal arts school for 18 months. He then worked part time at Atari to raise money for a trip to India in the summer of 1974, studying meditation and shaving his head. He also lived in a commune for a short time.

In 1975, he began associating with a group of computer aficionados known as the Homebrew Computer Club. Wozniak, a technical whiz, was trying to build a small computer, and Mr. Jobs became fascinated with its potential. In 1976, he and Wozniak introduced the Apple I, the first low-cost microcomputing system, for $666, and sold about 200 units. They followed that up in 1977 with the Apple II, a redesigned model that had impressive sales of $2.7 million in its first year. The company’s sales grew to $600 million by 1981 and created a market of personal computer users.

The Macintosh was introduced in 1984 with a commercial during the third quarter of the Super Bowl. The advertisement, designed by ad agency Chiat/Day and directed by Ridley Scott, fresh off his classic science-fiction film “Blade Runner,” ran just once, but it was named by Advertising Age as the commercial of the decade.

As controversial as he was creative, Mr. Jobs enforced a culture of secrecy at Apple’s Cupertino, Calif., headquarters. Mr. Jobs, who grew up idolizing the Hewlett-Packard ideal of an egalitarian workplace where employees were valued, was known in his younger years for playing pranks on underlings, reversing direction without acknowledging that he had changed his mind and yelling at company directors. He also refused to acknowledge paternity or pay child support for his first daughter for years. He threatened to sue teens who published Apple gossip on their Web sites.

In 1985, after tangling with John Sculley, the Pepsi executive he brought in to run the company, Mr. Jobs sold $20 million worth of stock and resigned from Apple. He and Wozniak had just received the National Medal of Technology from President Ronald Reagan. “I didn’t see it then, but it turned out that getting fired from Apple was the best thing that could have ever happened to me,” he said in his 2005 address at Stanford. “The heaviness of being successful was replaced by the lightness of being a beginner again, less sure about everything. It freed me to enter one of the most creative periods of my life.”

After several months, Mr. Jobs hired some of his former employees to begin a new computer company, called Next.

During that period, Mr. Jobs paid filmmaker George Lucas $10 million for a small computer animation firm and, over the next six years, he poured another $40 million of his own money into the company as it set out to make the first computer-animated feature film, “Toy Story.”

In late 1996, Apple bought Next for more than $400 million. Within months, Mr. Jobs was back at Apple as an adviser and quickly became chief executive again.

He stunned the faithful at a Macworld conference in August 1997 when he announced a partnership with Microsoft, accepting a $150 million investment in exchange for preloading Microsoft’s Internet Explorer browser on Apple computers. An unprecedented alliance between rivals, the deal ultimately saved the company by reassuring customers that they could use Microsoft’s ubiquitous Office software on Macs. “We want to let go of this notion that for Apple to win, Microsoft has to lose,” Mr. Jobs said, to shouts of dismay from his normally adoring audience. “Madman at the wheel, eh?” he added, laughing, as he walked off the stage.

That was just the start of his revival of the company. The “Think Different” advertising campaign got the world’s attention again, followed by the 1998 introduction of the colorful iMac desktop computer, which sold 1 million in a year. In 1999, the iBook appeared, a brightly colored, clam-shaped laptop that enabled wireless Web surfing.

Consumer releases came out in a rush. An overlooked technology called FireWire, a tool for quickly moving large amounts of data between digital devices, allowed the creation of iMovie, software that encouraged non-experts to make home videos. The iPod, an MP3-like player with room for thousands of songs, was introduced in October 2001 and has sold hundreds of millions of units. The iTunes application, which allowed consumers to legally buy and download music, started in April 2003 and revolutionized the digital music industry — more than 6 billion songs have been sold.

Mr. Jobs and Apple were suddenly cool again. The iPhone’s debut in 2007 generated a huge buzz, and it soon rolled over the competition from Palm Computing and BlackBerry, despite its higher cost. The iPad, a tablet-based computer introduced in January 2010, sold more than 10 million units its first year.

Although Mr. Jobs’s salary was $1 per year, his stock options made him rich. His fortune was estimated to be $5.4 billion by Forbes magazine in its 2008 survey of the richest people in the world.

But nothing came without controversy. Even as Apple’s stock was booming and its business thriving, Mr. Jobs faced scrutiny in a scandal about Apple’s backdating of stock options. Like many companies, Apple had given out stock options with effective dates chosen later, after it was known that the price was low on those dates, making the stock more valuable once it was sold. Mr. Jobs wasn’t the only employee who benefited, but the company said his options were approved at a special board meeting that never took place. The company was forced to restate its earnings, but a special company investigation concluded that Mr. Jobs had done nothing wrong.

Survivors include his wife since 1991, Laurene Powell; a daughter from a previous relationship, Lisa Brennan-Jobs; three children from his marriage, Eve Jobs, Erin Sienna Jobs and Reed Paul Jobs; and two sisters, Mona Simpson and Patti Jobs.

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