Internal product roadmaps are used to coordinate across your engineering, marketing, sales, and support teams. They typically contain more detail than a public product roadmap to inform your teams of the effort and activities required to release.
Public product roadmaps tell customers when they can expect features to be deliverd. A public roadmap is a valuable resource for your customers. It allows your customers to plan accordingly and make informed buying and building decisions.
It's important to understand your audience when developing the product roadmap. The roadmap is not a project plan. Decide early on how much detail is appropriate for your audience. For example, you might include more detail about the activities that need to be complete before release day while your customer facing roadmap will focus more on the features and timeline for when they'll be delivered.
Your roadmap should include a timeline for when new features will be delivered. Typically, you'll want to look three releases out as shown in the example below. This view provides Architects and other stakeholders enough time to plan accordingly.
Roadmaps are not release notes. Provide enough detail for your audience to understand what you are working on, but provide a disclaimer communicating that the plans may change.