Jobs, 55, has been instrumental in turning Apple into the dominant producer of portable music players, a leader in the smart phone business and, with the iPad, the inventor of a new category of modern tablet computers. He is Apple's public face, its master showman and its savior since he returned in 1997 after a 12-year hiatus to rescue the company from financial ruin.
Investors in recent years have pinned much of their faith in the company on Jobs himself, sending shares tumbling on every bit of news or rumor of his ailing health.
Apple did not say how long Jobs would be on leave. The company also did not provide any further information about Jobs' current condition, including whether Jobs is acutely ill, whether the leave is related to his 2009 liver transplant or whether he is at home or in a hospital.
Apple spokesman Steve Dowling referred to the text of Jobs' note to employees, which was made public Monday. The majority of Apple's board members did not immediately return messages seeking comment. Millard Drexler, CEO of J. Crew Group, was unavailable for comment according to a spokeswoman. Former U.S. Vice President Al Gore referred a reporter to Apple's press office.
In the note, Jobs said he will continue as CEO and will be involved in major decisions. Chief Operating Officer Tim Cook will be responsible for all day-to-day operations.
Cook, who is seen as a logical eventual successor to Jobs, is no stranger to the role. He ran the Cupertino, Calif.-based company for two months in 2004 while Jobs battled pancreatic cancer, and again in 2009 during Jobs' most recent medical leave. Apple ran smoothly then, releasing a new version of the iPhone and updated laptops on schedule.
Since Cook began with Apple in 1998, he has been credited with tuning Apple's manufacturing process to solve chronic product delays and supply problems.
But Cook's presence at the helm may not be enough to cushion the blow to Apple's shares. The company is likely wrapping up work on a second generation of the iPad and competition among tablet makers is expected to heat up this year and next.
In 2010, investors seemingly grew accustomed to Jobs' extreme thinness, focusing instead on the early success of the iPad with consumers. But they may still fear that without Jobs, Apple could lose its early lead, or in the longer run cede its role as the high-end, high-design trend setter of the PC and consumer electronics industries.
"Steve is clearly still the visionary behind Apple," said Tim Bajarin of Creative Strategies, who has been covering Apple for decades.
But, the analyst said, Apple is a well-run company at the top of its game.
"I think investors have to take a hard look at where Apple is and where Apple will continue to go given the fact that it has incredibly strong leadership. All the products that Apple's going to bring out in the next two years are already in the pipeline," he said.
The company announced Jobs' leave a day before the company is set to report quarterly earnings. U.S. stock markets were closed for the Martin Luther King Jr. holiday.
In Europe, investors reacted sharply and Apple's shares closed in Frankfurt a staggering 6.6 percent lower at 243 euros ($323.02).
Apple has a history of extreme secrecy when it comes to the iconic CEO's health, disclosing major illnesses only after the fact.
The company waited until after Jobs underwent surgery in 2004 to treat a very rare form of pancreatic cancer -- an islet cell neuroendocrine tumor -- before alerting investors. That type of cancer can be cured if diagnosed early, unlike the deadlier and more common adenocarcinoma.
By 2008, Jobs had lost a noticeable amount of weight, but Apple attributed his gaunt appearance to a "common bug." In January 2009, Jobs issued a statement saying the weight loss was caused by a hormone imbalance, and that the treatment was simple. He backtracked less than two weeks later, however, announcing a six-month medical leave. During that time, he received a liver transplant that came to light two months after it was performed.
"I love Apple so much and hope to be back as soon as I can," Jobs wrote in Monday's note to employees. "In the meantime, my family and I would deeply appreciate respect for our privacy."