Apple COO Tim Cook on stage last week at an event to announce the Verizon Wireless version of the iPhone.

As Apple CEO Steve Jobs takes another medical leave of absence, he again leaves the company he founded in the care of his chief operating officer, Tim Cook. So who is Cook

A reserved and private man, Cook has been thrust into the spotlight for the third time in eight years, taking over temporarily for what many would say is the irreplaceable Jobs.

A former Compaq executive, Cook joined Apple in 1998 as a senior vice president of worldwide operations. He was promoted to chief operating officer in 2004. Before Compaq, Cook also spent 12 years at IBM, where he ran manufacturing and operations for the company's PC business.

Known for completely restructuring Apple's manufacturing operations, Cook insisted that Apple shut down its overseas factories and farm out the work to third-party manufacturers. As a result, the company has cut down its inventory and improved margins on its entire product lineup. He's intensely focused on cutting costs as well, and though far from the leading computer maker in the world by volume, Apple is known for reaping the most profits per computer it produces.

Since Cook's last time subbing for an ailing Jobs two years ago, Apple has better than doubled its cash reserves to more than $50 billion.

Though Apple refuses to publicly comment on its eventual succession plans for who will head the company after Jobs leaves someday, Cook is regarded by some as the logical choice. He doesn't have Jobs' charisma or knack for knowing what customers want in a phone or computer product, but he runs the company in the way Jobs has long envisioned.

Cook's profile has grown steadily in the last few eyars. He is Jobs' right-hand man at shareholder events, and has completely handled earnings calls with analysts for last few years. Recently he has played a more prominent role at product launches. Perhaps foreshadowing today's news, it was Cook and not Jobs who flew to New York last week to join Verizon COO Lowell McAdam at the high-profile launch of the first iPhone on Verizon's network.

Apple is scheduled to report its first-quarter fiscal 2011 earnings tomorrow after 1 p.m. PT.