Gartner defines Unified Communications (UC) as those that facilitate the interactive use of multiple enterprise communications methods. This can include control, management and integration of these methods. UC products integrate communications channels (media), networks and systems, as well as IT business applications and, in some cases, consumer applications and devices.
In essence, UC provides systems that combine your desktop, desk phone, mobile phone and tablets into a common communication platform that allows users to collaborate with one another using a combination of Video, Voice, Messaging and Whiteboarding.
An important part of UC today is UCaaS (Unified Communications as a Service) that basically offers a hosted UC solutions for enterprises that do not want to incur the cost and complexity of a locally hosted UC solution.