A project plan, according to the Project Management Body of Knowledge, is: "...a formal, approved document used to guide both project execution and project control. The primary uses of the project plan are to document planning assumptions and decisions, facilitate communication among stakeholders, and document approved scope, cost, and schedule baselines. A project plan may be summarized or detailed."
- "...a statement of how and when a project's objectives are to be achieved, by showing the major products, milestones, activities and resources required on the project."
The project manager creates the project management plan following input from the project team and key stakeholders. The plan should be agreed and approved by at least the project team and its key stakeholders.
At a minimum, a project plan answers basic questions about the project:
- Why? - What is the problem or value proposition addressed by the project? Why is it being sponsored?
- What? - What is the work that will be performed on the project? What are the major products/deliverables?
- Who? - Who will be involved and what will be their responsibilities within the project? How will they be organized?
- When? - What is the project timeline and when will particularly meaningful points, referred to as milestones, be complete?
Plan contents 
To be a complete project plan according to industry standards such as the PMBOK or PRINCE2, the project plan must also describe the execution, management and control of the project. This information can be provided by referencing other documents that will be produced, such as a Procurement Plan or Construction Plan, or it may be detailed in the project plan itself.
The project plan typically covers topics used in the project execution system and includes the following main aspects:
- Scope Management
- Requirements Management
- Schedule Management
- Financial Management
- Quality Management
- Resource Management
- Communications Management
- Project Change Management
- Risk Management
- Procurement Management
It is good practice and mostly required by large consulting and professional project management firms, to have a formally agreed and version controlled project management plan approved in the early stages of the project, and applied throughout the project.
See also 
- PMBOK, 2000 Edition