Four years into its existence as an Internet phenomenon, Twitter has finally clarified how it will make money. The general approach was never in doubt: It will sell ads, but the specifics come as a surprise.

In a blog post on Tuesday, company co-founder Biz Stone introduced an advertising program called Promoted Tweets.

"You will start to see Tweets promoted by our partner advertisers called out at the top of some Twitter.com search results pages," said Stone. "We strongly believe that Promoted Tweets should be useful to you. We'll attempt to measure whether the Tweets resonate with users and stop showing Promoted Tweets that don't resonate."

Initially, Promoted Tweets will come from a select group of advertising partners: Best Buy, Bravo, Red Bull, Sony Pictures, Starbucks, and Virgin America.

The company expects to add additional advertisers to this trial phrase group and later to open Promoted Tweets to other advertisers and to Twitter partners, such as makers of Twitter clients.

During this second phase, the company plans to add Promoted Tweets to users' timelines "in a way that's useful to you," Stone explained.

Promoted Tweets differ from Google text ads in that they're integrated into Twitter's search results rather than segregated in an area reserved for ad copy.

Only one Promoted Tweet will be presented on each search results page and each will be clearly labelled, Stone says.

Twitter's goal is to deliver sponsored content that's timely, relevant and useful.

In a blog post, John Battelle, co-chair of the Web 2.0 Conference, search expert, and founder and chairman of Federated Media Publishing, says that Promoted Tweets will mark the first time that Twitter users see a tweet from someone they have not specifically chosen to follow.

He argues that the new program should be welcomed by Twitter developers and that it could follow the path to success trodden previously by Google's AdWords.