They had their eye on Aquantive, and after laying the deal before the SEC and writing up a deal with the company, they have closed the deal. For $6 Billion, the existing board of Aquantive will step down. On top of that, the company will cash out their stock and become owned by Microsoft.
What will Microsoft do with their new toy? Probably not much at first, though whatever it is they do I doubt we will hear about it. The back-end of advertising is something that often goes largely unnoticed, though clearly Google will take note of Microsoft's actions.