Twitter has acquired desktop client TweetDeck, the companies have confirmed.

"I am extremely happy and proud to let you know that TweetDeck has been acquired by Twitter," TweetDeck founder Iain Dodsworth announced on his company's blog today. "We completed the deal on Tuesday and are now in the process of 'joining the flock.'"

Twitter's acquisition of TweetDeck, a favorite among Twitter users, isn't all that surprising. Since last month, reports have been swirling that the companies were in negotiations. And earlier this week, CNN reported that the social network had agreed to acquire TweetDeck, which provides a "personal browser" for staying in touch with updates from users on Twitter, as well as Facebook, MySpace, and LinkedIn, for $40 million in cash and stock.

Neither Twitter nor TweetDeck announced the terms of their deal.

TweetDeck is arguably one of the most useful tools available in the social-networking ecosystem. The platform provides simplified views of a respective user's social networks to keep them apprised of what's happening in their friends' lives. Even more importantly, it saves them time. With the help of TweetDeck, users can update their Twitter, Facebook, MySpace, and LinkedIn profiles from the app. They can also check in to locations via Foursquare. A scheduling option lets users send out tweets at a desired time in the future.

TweetDeck currently offers free iPhone and Android apps to users. The company is also working on an iPad option.

Though Twitter didn't say in detail what it has planned for TweetDeck, the company did point out that it will use the service to help commercial users track conversations across the social network.

"TweetDeck provides brands, publishers, marketers and others with a powerful platform to track all the real-time conversations they care about," the social network wrote on its blog. "In order to support this important constituency, we will continue to invest in the TweetDeck that users know and love."

In an interview with All Things Digital's Peter Kafka, Dodsworth emphasized that TweetDeck will continue to exist as a standalone product. "From a technical standpoint we'll move towards become part of the platform. They won't be shutting it down, they are in fact investing further in its future," he said.

And Dodsworth expects TweetDeck to continue to support multiple social network platforms. "The reality of it is that TweetDeck usage has been heavily Twitter-based with the external services not heavily used but acting more as a value-add for our users. I can't see them going away anytime soon," he told All Things Digital.

Twitter's TweetDeck acquisition is the latest in a string of strategic purchases the company has made over the last several years.

In 2008, Twitter acquired Summize, a Twitter search engine that became the basis for the current Twitter Search. Last year, it acquired a small start-up called Cloudhopper to help it effectively handle the growth of SMS tweets around the world. Also last year, it acquired Atebits, which was operating a highly acclaimed, paid mobile-tweeting client Tweetie. That application was made free and is now the official Twitter app for mobile users.

This post was updated at 9:28 a.m. PT with more details and again at 10:08 a.m. PT with quotes from TweetDeck's CEO.