Yang, who has held the post of Chief Yahoo since Semel took over in 2001, has taken over as CEO, relegating Semel to his non-executive chairman’s position.

Sue Decker, former CFO, current executive vice president and often tipped as Semel’s heir apparent, has been named president of the online services firm.

In prepared statements, much was made of Yang and Decker as an executive partnership, rather than playing up the newfound importance of either person over the other.

Semel took the reins at Yahoo six years ago, a year after the dot-com bubble had started to deflate, and he managed to steer the company through the rough times as the portal age came to an end, leaving the likes of AltaVista and Excite by the wayside, and into the age of search.

Today, we are again addressing challenges created by dramatic changes in the needs of audiences and advertisers, Semel said in a letter to the Yahoo board yesterday. And of course none of us is at all satisfied with the company’s recent financial performance.

The return of Yang, who co-founded Yahoo with David Filo in 1994, invites comparisons with the return of Michael Dell at Dell earlier this year.

Like Dell, Yahoo had come under scrutiny over the perception that it was internally stagnating and failing to capitalize on opportunities and respond to change, and that a fresh vision and organizational rethink was required.

In Yahoo’s case, comparisons to Google, which despite having issues of its own is generally seen as an unstoppable and yet still-nimble growth machine, were inevitable.

Stories of Yahoo’s failure to absorb acquisitions, leading to an eclectic portfolio of disparate services, and its dragging of feet when it came to revamping its ad platform have become staple stories in the Silicon Valley rumor mill of late.

And Yang’s vision for a turnaround? He wrote on the corporate blog: A Yahoo! that executes with speed, clarity and discipline. A Yahoo! that increases its focus on differentiating its products and investing in creativity and innovation. A Yahoo! that better monetizes its audience. A Yahoo! whose great talent is galvanized to address its challenges. And a Yahoo! that is better focused on what’s important to its users, customers, and employees.