In the new digital series World 3.0, Yahoo Global News Anchor Katie Couric introduces you to ideas that are changing the world and the people behind them. Leaders in technology, health, philanthropy and business share the secrets of their success and the innovations that are improving, and in some cases disrupting, their industries.

By Katie Couric

These are the two magic words that have led to some of the most profound and game-changing innovation in history: "What if?" And with my new digital series "World 3.0," I'll speak with some of the men and women who have asked that question in tech, science, medicine, business and philanthropy and have found answers that improved — and in some cases disrupted — their industries.

Brian Chesky is one of those people.

He was, by his own description, broke and unemployed, struggling to pay the rent on his San Francisco apartment with his roommate Joe Gebbia. Then it hit them. What if they could rent some of their apartment space to a few people in town for an event, such as an international design conference?

That was in 2007, and it was the beginning of a boom for what is called the sharing economy. Bikes, cars even dogs (if you can believe that) can be rented on a temporary basis. So why not a bed, an apartment or even a castle in Ireland?

Today, the company that Chesky founded in 2007 with Gebbia and Nathan Blecharczyk, Airbnb, is valued at $10 billion dollars and has rental listings in 192 countries. Now, Chesky says he asks the question, "What if we'd never taken the chance?" His personal stake in the company will make him, at 32 (33 in August, as one employee pointed out), one of the world's youngest billionaires.

The success he's enjoyed has been as enormous as the 72,000-square-foot office space Airbnb calls home but it hasn't come without challenges. There have been some high-profile incidents of apartments being ransacked, trashed or used for sex parties. Chesky admits that in the beginning his company may have been a bit cavalier, if not outright foolish, in its approach to security. After a major incident in 2011, Airbnb implemented insurance policies and now has 100 employees devoted exclusively to trust and safety. Still, from time to time trouble arises. Just this past April, I read about Airbnb apartments being turned into brothels by prostitutes in New York City.

Chesky points out that while these kinds of mishaps have happened, Airbnb has more than 600,000 listings and has had millions of guests who've rented from hosts, with no problems.

A more pressing question is how many of his 600,000 listings are actually legal. From Barcelona to Paris to Malibu, municipalities are challenging the right of hosts to rent their properties and investigating landlords who list multiple units at once. At stake are millions of dollars in lost tax revenue, not to mention certain regulations and codes that these amateur hoteliers may be circumventing.

In New York City, it is estimated that as many as half of Airbnb's listings may be illegal, breaking a law that prohibits rentals of fewer than 30 days. If you haven't been following the court battle with New York state Attorney General Eric Schneiderman, the punch line is Airbnb has handed over data on its customers, and the state will investigate anyone who seems suspicious. So stay tuned.

Right now, the stakes couldn't be higher. I met up with Brian Chesky at a critical moment. If enough of the listings on Airbnb around the world are shuttered for legal reasons, his business could go bust just as he begins to expand.

A few months ago, rumors began to swirl about a top-secret one-page memo that contained the details of Airbnb's plot to take over the hospitality industry. It sounds very Dr. Evil, right? In my interview, I pressed him on the details of his plan, and while he was a little cagey, I'd say it's clear that the list of big businesses that hate him will soon be longer than just hotels. But he did joke that there are no plans to launch an airline. Yet.

I think his story, the success his company has had and the place it holds in this rapidly growing sharing economy make him the perfect person for the launch of "World 3.0." I'm really excited by the possibilities blossoming in tech today, all those "what ifs" being set in motion.

That's a big part of why I wanted to launch this series, and why I joined Yahoo. I am answering my own "what if" after more than three decades in television.

The world is changing, or iterating, as they say in this new world. I'm thrilled to be part of it and to tell you the stories of the people and ideas that are part of our future.

Who do you think is a global changer, and who would you like to see featured in this series? Let me know on Twitter (@katiecouric) or on Tumblr.