A communication plan is a policy-driven approach to providing stakeholders with information. The plan formally defines who should be given specific information, when that information should be delivered and what communication channels will be used to deliver the information.
The purpose of a Communication Plan is to define who needs to be aware of and informed about the project, how and how often information will be distributed, and who will be responsible for the distribution. It is a part of the overall Project Management Plan.
While those involved in the planning effort may be well aware of what is going on, those outside the team are often uninformed and uninvolved. Effective communication ensures that all members of the organization are aware of the plan, its importance and how they might be impacted.