Communication is a spectrum of capabilities designed to increase productivity and collaboration among users across a variety of mediums. Including
Presence is a key component of Unified Communications. It enables you to find the best person available for live contact, and also shows you the state of the receiver so you can choose the best or richest mode of communications.
Mobility is the ability to communicate using any device no matter where you are.
Unified Messaging is about the integration of e-mail, voicemail, and ultimately video mail, using existing enterprise contact directories as a common database.
Video brings a new dimension to traditional voice calling by letting users "meet" face-to-face with remote individuals from wherever they happen to be.