By Miguel Helft, senior writer
FORTUNE -- With the resignation of Steve Jobs, Apple's ailing chief executive, his successor and collaborator of many years, Tim Cook, will be tested like never before.
Cook, who was Apple's chief operating officer, has been running day-to-day operations since January, when Jobs announced that he was taking a medical leave. And twice before, Cook had taken the reins from Jobs, as Apple's founder stepped aside to focus on his health.
Like Jobs, Cook is a relentless executive and exacting boss, a perfectionist who obsesses over minute details. But the similarities between the two men end there. While Jobs is a charismatic leader known for outbursts of temper, Cook, who was raised in a small town in Alabama, is soft-spoken, reserved and intensely private. And as Jobs used his creative genius and vision to conceive and design blockbuster products like the iMac, iPhone and iPad, Cook's considerable operational skills were focused on making sure that Apple could build millions of those products and deliver them to every corner of the world to meet customers' seemingly insatiable demand.
Their complementary skills, helped Apple (AAPL) engineer one of the most remarkable turnarounds in American history.
In a news release, Apple director Art Levinson, the chairman of Genentech, speaking on behalf of Apple's board, expressed confidence in Cook. "The Board has complete confidence that Tim is the right person to be our next CEO," Levinson said. And he said that in his new role as chairman, Jobs "will continue to serve Apple with his unique insights, creativity and inspiration."
Still, the unexpected resignation of Jobs is raising new questions about his health. In his resignation letter, Jobs said: "I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's C.E.O., I would be the first to let you know. Unfortunately, that day has come."
Jobs, who has battled pancreatic cancer since 2004 and underwent a liver transplant two years ago, did not discuss his health, and an Apple spokesman declined to comment.
By all accounts, Cook , who is 50, has done an admirable job of steering Apple in Jobs's absence. But as he assumes the CEO role on a more permanent basis, Cook will have to prove that he can succeed without Jobs at his side.
The test, however, is not likely to come immediately. While investors may be spooked by Jobs's resignation, the company has been on a remarkable, multi-year streak, built on the success of the iPhone and iPad and the resurgence of the Macintosh. That's not likely to end overnight.
"Apple does not create products on a quarter by quarter basis," said Tim Bajarin, president of Creative Strategies, who has followed Apple for more than three decades. "It has products in development through 2013 and a roadmap through 2015. Everything that is in the works for the next five years has Steve's imprint and blessing."
And while it is impossible to overstate the influence of Jobs, Apple is not a one-man operation. The company has a deep bench of seasoned executives, who have absorbed Jobs's vision and direction. They include Philip W. Schiller, the company's marketing chief; Jonathan Ive, the London-born designer who is Apple's senior vice president for industrial design; and Scott Forstall, a senior vice president for iOS software.
Without Jobs in the mix, you can expect Cook to rely more heavily on them.
Apple's succession plans have been a closely guarded secret. Still everyone from Silicon Valley to Wall Street had expected that Cook would eventually replace Jobs. Now we know. And it seems clearer than ever that Jobs had planned for this day -- and for his gradual exit from Apple -- with the same care and precision he applied to the release of a new product or one of his keynote speeches.>
More on Steve Jobs and Apple from Fortune:
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