Microsoft® Lync® is an enterprise-ready unified communications platform. With Lync, users can keep track of their contacts’ availability; send an IM; start or join an audio, video, or web conference; or make a phone call—all through a consistent, familiar interface. Lync is built to fully integrate with Microsoft Office.
Also note, that Lync is versioned and updated separately from the rest of the Office 2011 suite.