The process of determining the information and communications needs of the project stakeholders: who they are, what is their level of interest and influence on the project, who needs what information, when will they need it, and how it will be given to them. [PMBOK]
The objective of this process is to make sure that project team members, customers and stakeholders have the information they need to do their jobs. Proactive communication is important on all projects. Communication is also a vital way to manage stakeholder's expectations about how the project is progressing and who needs to be doing what.
A Communication Plan allows the project manager to think through how to communicate most efficiently and effectively to the various constituents. Communication needs to be in the right format, to the right target audience with sufficient amount of information and at the right time.