In project management there will be a number of project plans like the risk management plan, the change management plan, and the communications management plan. All of the different plans put together is your project management plan.
So often the project schedule is referred to as the project plan but technically the project plan is the combination or integration of all the different project management plans. The project charter is also a key document to create the project management plan.
The project management plan will describe how the project will work and be conducted. How the project will be executed, monitored, and controlled. One of the most common outputs for any project management process is updating the project management plan.
The project management plan should also be used as the project’s baseline so when changes are made to scope for example, the project management plan will have the original requirements.
The project management plan is the integration of all other project plans (i.e. risk management plan, change management plan, communications management plan, etc.) and can be a high level summary or it can be in detail.