Unified communications (UC) is the integration of real-time communication services such as instant messaging (chat), presence information, telephony (including IP telephony), video conferencing, data sharing (including web connected electronic whiteboards aka IWB’s or Interactive White Boards), call control and speech recognition with non-real-time communication services such as unified messaging (integrated voicemail, e-mail, SMS and fax). UC is not necessarily a single product, but a set of products that provides a consistent unified user interface and user experience across multiple devices and media types.
In its broadest sense Unified Communications can encompass all forms of communications that are exchanged via the medium of the TCP/IP network to include other forms of communications such as Internet Protocol Television (IPTV) and Digital Signage Communications as they become an integrated part of the network communications deployment and may be directed as one to one communications or broadcast communications from one to many.
Unified Communication allows an individual to send a message on one medium and receive the same communication on another medium. For example, one can receive a voicemail message and choose to access it through e-mail or a cell phone. If the sender is online according to the presence information and currently accepts calls, the response can be sent immediately through text chat or video call. Otherwise, it may be sent as a non real-time message that can be accessed through a variety of media.
Source : Wikipedia