Jobs announced Wednesday in an e-mail to Apple employees that he would to deal with lingering health issues. That thrust Cook--largely unknown outside of Apple--into the interim CEO post.

"I know he and the rest of the executive management team will do a great job," Jobs told employees.

While Cook has maintained a low profile since becoming Apple's COO nearly four years ago, he has a strong reputation in the business press for keeping the company running smoothly outside of the limelight. A called him a "low-key operator" who helped fix the manufacturing inefficiencies that dogged Apple in the mid- to late '90s. Cook, 48, also has a reputation for being detail-oriented.

He's also familiar with stepping in for Jobs. When the Apple CEO was treated for pancreatic cancer in 2004, it was Cook who took over the day-to-day operations then.

Cook participates in Apple's quarterly conference calls with analysts, along with chief financial officer Peter Oppenheimer. He also sits on the board of directors for Nike--the only executive at Apple other than Steve Jobs to sit on another company's board, according to from last November.