It's simple: He's the guy that makes Apple run. Even simpler: He's CEO Tim Cook's Tim Cook.
FORTUNE -- Looks aren't everything. Yes, Apple's devices are examples of uncompromising industrial design, but all the style in the world won't matter if that snazzy new iPhone, iPad, or iMac isn't manufactured, shipped, and delivered on time. And that's where Jeff Williams, Apple's senior vice president of operations, comes in.
One of new CEO Tim Cook's most trusted lieutenants, Williams, 48, essentially does a big chunk of what Cook did when he was chief operating officer. Williams manages the company's vast supply chain and production process, including overseeing Apple's relationship with supplier Foxconn, which churns out iPad 2 tablets and other fare from its facility in Chengdu, China. He negotiates supplier deals, keeps production going, and makes sure devices get where they need to be, when they need to be there -- all while keeping costs down and without sacrificing quality.
A health-minded cyclist (like Cook), Williams is known as an intensely private, salt-of-the-earth kind of guy. According to Apple lore he drove around in a beat-up Toyota with a broken passenger-side door even after he was appointed to management. Those who know him say he's even-keeled and direct, a good coach who will go over a problem with employees, explain what they need to do better, and move on.
As a mechanical engineering major at North Carolina State University, Williams spent a week in a training program at the Center for Creative Leadership in Greensboro, N.C. There he examined his strengths, weaknesses, and interactions with others. The program left a lasting impression: He now sends Apple (aapl) middle managers to similar courses. After college, he worked at IBM (ibm) and picked up an MBA from the same Duke University evening program as Cook. (The two didn't overlap.) He joined Apple in 1998 as head of worldwide procurement.
"With Jeff, what you see is what you get," says Gerald Hawkins, director emeritus of the Caldwell Fellows program at North Carolina State and a friend of Williams's. "And if he said he was going to do something, well, he'd do it."
Williams has done a lot for Apple, all of it behind the scenes, such as his work on a deal to prepay suppliers like Hynix some $1.25 billion for flash memory, a move that helped Apple launch the Nano.
He also worked on ways to streamline the iPod delivery process, says Steve Doil, an ex-Apple employee who worked with Williams in the operations group. That explains why U.S. consumers can go online, buy an iPod, have it engraved, and receive it within three business days from start to finish.
His attention to the small stuff will serve him well in the years to come. Because, while Cook may have some pretty big shoes to fill, so does Williams.
This article is from the September 26, 2011 issue of Fortune.