Hi Sam,
what helped me while studying for the PMP exam was keeping in mind what each of the two documents is meant for.
The project charter contains high-level information on the project (and the product) and is the document that formally authorises the project, and gives the PM the authority to work on the project, managing project activities and resources needed. As such, it needs to include the name of the PM. When you look at this document, you should be able to get a general picture of the project.
The project management plan details how the project will be executed, monitored & controlled, and closed. So whenever you need to know how to manage something in your project, you will refer to the project management plan. For this reason, it will include (among other things) all management plan from the corresponding knowledge areas (scope/schedule/cost/quality/resource/communication/risk/procurement/stakeholder) + requirements mgmt plan + change mgmt plan + configuration mgmt plan.
Now, suppose you need a high level description of the project, including its purpose and any elements to decides whether the project has been successful or it has to be terminated. Where would you look for this info? I would go for the project charter: you have to define all these elements in advance, so that when the project is authorised you have all this info available. And, most importantly, they will be the basis for the development of all the subsidiary mgmt plans; you may think of it as a starting point for them. Which is why the project charter has to include also key stakeholders, summary milestone schedule, pre-approved financial resources, and overall project risk.
Hope this helps,
Gabriella
