The project description is simply that – a brief description of the project and what it encompasses.  This section of your project management plant should be clear and concise but provide enough information that anyone reading it can get a high level view and understanding of your project.  It should define the purpose, scope and objectives of the project as well as product deliverables, estimated time frame to completion and the cost or budget for the project.  Basically, it should briefly describe the 4 W’s:

Who?

What?

When?

Where?

You can think of it as an overview and further information for each of these areas can be found in more detail either later in your document or in ancillary documents to support the PMP.