The project plan is the mother of all documents for your project, the one that documents everything from initiation to closing. It includes several subsidiary plans gathered from all knowledge areas, including the schedule.
The project plan is the go-to document to manage your project until the very end, and it’s the document you want to refer to if you inherit an ongoing project.
The project schedule is part of the project plan, and includes all your project’s important dates, including milestones like a BETA deliverable for example.
The format may vary from a bar chart, to a Gantt chart, to a calendar, but the important thing to remember is that the schedule = dates!
The confusion between those two terms can bring conflict between two colleagues if one asks for a project plan and simply receives a schedule, so it’s important that those terms are the same for everyone.
Hope this helps!