Ballmer, 43, is a larger-than-life character known for his hyperkinetic, booming voice, his penchant for basketball and his ruthless desire to expand Microsoft's market share. He has been credited as being one of the architects of the company's far-flung yet intricate sales operation.
In the mid-1990s--when Microsoft was cementing deals and whipping up industry support for Windows NT and Windows 95--Ballmer was often the one they sent on the road.
To Wall Street, Ballmer is a familiar figure. Since he's been Microsoft's president for more than 18 months, analysts stated that the transition from Gates to Ballmer should be relatively painless.
"He is clearly one of the three or four most important people who have been associated with Microsoft probably since its birth," Hambrecht and Quist analyst Christopher Galvin said this past June.
"What (Ballmer) is particularly good at is creating a powerful sales organization and (sending a) strong message to the marketplace. I think the customer-oriented restructuring was emblematic of that."
Others also expect a smooth transition.
"Ballmer is a terrific manager and leader," said William Epifanio, a J.P. Morgan Securities analyst. "He's been president for a while, so we expect this will be a low-risk transition."
Industry observers have said one of Ballmer's chief accomplishments has been his efforts to get some 30,000 employees to make customer satisfaction a priority. Last March, Ballmer led the push to transform Microsoft from being organized by product groups to units focusing on different product segments.
In certain respects, Ballmer and Gates are polar opposites. While Gates often fits the part of a fastidious intellectual, Ballmer comes across as a glad-handing schmoozer who is comfortable in a crowd. The contrast between the two executives has been the subject of a number of Microsoft corporate videos.
Still, strong similarities exist. Like Gates, Ballmer is known for his intensity and argumentative style.
Ballmer met Gates at Harvard in the 1970s. After completing a degree in applied mathematics, Ballmer took a job at Procter & Gamble. In 1980, he dropped out of Stanford Business School to join Gates at Microsoft. Since then he has held a number of positions at the company.
News.com's Ben Heskett contributed to this report.