By Brian A. Shactman, CNBC Reporter

NEW YORK ( CNBC) -- It is time to ask and answer the question once again: Who is Tim Cook?

With Apple ( AAPL) CEO Steve Jobs stepping back for an unknown amount of time, Cook is stepping in for the third time since he joined Apple in 1998.

The last time this happened was two years ago. In mid-January of 2009, Jobs took leave, and Cook took the post of interim CEO.

Initially, the stock sold off about 10 percent. That loss was reversed within days, and in the six months that Jobs was dealing with a liver transplant, the stock rose 70 percent, far outpacing the broader indexes.
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Of course, it must be noted that the stock has added another 134 percent since Jobs return on June 29, 2009.

This time, Jobs has not relinquished the CEO title, but Cook is now experienced with this type of situation. He also filled in for Jobs in 2004.

As for Cook's background, he has had stints at both IBM ( IBM) and Compaq and was one of Jobs' first hires when he returned to the company in the late 1990s. Cook became Chief Operating Officer in 2005.

His strength is in operations and not product design or marketing, but when it comes to operations, he is considered one of the best in the world.

Cook is widely credited with streamlining the supply chain, managing inventories extremely well and helping Apple stockpile about $50 billion in cash and short-term investments.

However, if there is a scenario in which Jobs does not return, Cook won't be alone at the dais, even if he ascends to the CEO post.

When Jobs was gone in 2009, Cook did not lead the show when the iPhone 3GS was unveiled. But he did front last week's ceremony with Verizon ( VZ) when the wireless carrier announced the addition of the iPhone to its network.

One name to keep an eye on is Jonathan Ive, who is an executive on the design side of the business.

-- Written by Brian A. Shactman of CNBC
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