Microsoft's system for unified communications in the enterprise. It includes instant messaging (IM), voice over IP (VoIP) and Web conferencing both within the organization and externally. Previously Office Communications Server (OCS), which evolved from Live Communications Server, Lync Server uses the address books maintained by Microsoft Exchange Server.
Microsoft Lync is the instant messaging client for Lync Server. It was formerly Microsoft Office Communicator (MOC), which replaced Windows Messenger. See Skype for Business