What’s an Internet company like Yahoo doing with Katie Couric? And what’s Couric, one of TV’s best-known news personalities, doing with Yahoo?

The online giant and the former “Today” show host formalized their courtship on Monday. Couric, currently employed by ABC News, will log on to Yahoo in January as its “global news anchor,” an impressive but vague title whose full dimension hasn’t quite been worked out.

The multi-service Yahoo, which offers news, finance, e-mail and Web search, already rivals Google as the most heavily visited destination on the Internet. What it lacks is “engagement,” the term for repeated and prolonged use. That’s where Couric, 56, may come in.

Couric’s hiring fits with Yahoo chief executive Marissa Mayer’s strategy of using news to create a daily habit for millions of Web users. Mayer, who became the company’s fifth chief executive in as many years in 2012, has been on a buying spree, picking up app developers and companies such as the microblogging platform Tumblr, all in hopes of keeping users coming back and sticking around longer every day.

Couric gives Yahoo, and its Yahoo News subsidiary, an instantly familiar face and a personality around which to build its already massive monthly audience of more than 190 million unique visitors.

The former “CBS Evening News” anchor isn’t the first TV personality to make a move to the Web — Glenn Beck and Larry King have their own Web operations — but having her in Yahoo’s portfolio is another way to attract advertisers interested in video ads, which are more lucrative than the traditional text-based kind on the Web.

Yahoo is “kind of a shell,” said Colin Gillis, senior technology analyst at BGC Partners. “Now they’re filling it up.”

“It’s expense before revenue,” Gillis added. “This is part of the equation [for Yahoo], but it’s the easy piece: writing the check.”

Indeed, for the moment, Couric’s hiring looks as much like a headline-grabbing stunt than a transformative change. Yahoo said Couric will conduct “newsmaker” interviews that will stream on Yahoo’s home page along with other, unspecified reporting roles. But it did not say how often she will appear on the site.

“We’re still getting to the bottom of how broad [Couric’s] scope will be,” said Yahoo spokeswoman Carolyn Clark. Couric would be “fully committed” to Yahoo, Clark added, and would work with Megan Liberman, editor in chief of Yahoo News, but she could not offer specifics.

In fact, Couric could actually have a more limited role at Yahoo, at least in the immediate future, than she does now. Couric says she’s not giving up her syndicated TV talk show, “Katie,” even as she joins Yahoo. The weekday program is in its second season and has performed relatively well in the ratings. Although the show faces an uncertain future — producer Disney-ABC hasn’t decided whether to renew it — Couric is locked in through at least May, leaving limited time for other projects.

Yahoo now posts weekly clips from Couric’s show, as part of a two-year-old “content-sharing” partnership with ABC News.

Yahoo didn’t disclose the financial terms of its agreement with Couric, who is leaving ABC News before her current three-year deal is over.

Couric is the biggest name among a series of journalists whom Mayer has added in recent weeks to Yahoo News, long known more as an aggregator of news from other sources than a creator of original work.

Neither Couric nor Mayer was available for comment.

Mayer recently raided the New York Times for three prominent journalists, including Liberman, a deputy Times editor brought on by Yahoo in September. David Pogue, the Times’ popular tech reviewer and columnist, joined last month, as did Matt Bai, the chief political correspondent for the New York Times Magazine. (The Times appears to be a popular place for Yahoo recruiters; Yahoo also hired politics and culture writer Virginia Heffernan from the Times last year.)

The site has occasionally distinguished itself, particularly with its sports reporting and opinion columns. It also has ramped up its Washington coverage and was a constant presence on the campaign trail last year.

Yahoo once harbored dreams of challenging Google as the dominant site on the Internet. But the gap between the two — at least in revenue and profit — has widened over the years as Yahoo churned through a series of strategies and chief executives and Google dominated the search business. Over the past dozen years, Yahoo has made more than $14 billion in acquisitions, several of which proved disastrous. (Google had $50.2 billion in revenue and a $10.7 billion profit last year, compared with Yahoo’s revenue of $4.99 billion and operating profit of $566 million.)

Yahoo had been a classic Silicon Valley success story; it was started in 1994 by Jerry Yang and David Filo while they were Stanford University engineering students. What began as a directory of Web sites soon grew into a global Internet portal, offering a variety of digital services and information.

Although laid low by the high-tech bubble of 2000-2001, the company rebounded under former chief executive Terry Semel, the former co-chief executive of Warner Bros. Semel tried to expand into original video production and built a massive campus near the Los Angeles airport. With the recession’s onset, Yahoo began to stumble again and went through a period of rapid turnover at the top.

At one point in 2008, it was the target of a $44 billion takeover attempt by Microsoft, a deal it rejected.

Under Mayer, one of the few female chief executives of a Fortune 500 company, Yahoo has rebounded strongly; its stock has more than doubled since October of last year.

Gillis, the stock analyst, said that offering one-of-a-kind content, particularly news content, gives Yahoo a way to get people to return regularly and to interact with the other features and information that Yahoo offers. Greater time spent on a Web site translates into advertising revenue, something Mayer will eventually need to show to justify all of Yahoo’s recent investments.