A product roadmap describes the high level product direction along a time line. It contains feature/function enhancements and also describes a vision or strategy. The product roadmap provides guidance for internal and external stakeholders (e.g. customers, partners and development). The product backlog contains actionable steps to walk along a product roadmap.
Internal purpose
The product roadmap is a set of milestones for reaching strategic goals in the product. Development, marketing and product management align on the product roadmap.
External purpose
The product roadmap provides guidance to customers. Users and buyers can plan with future product functionality, engage with the product owners and provide feedback.
A product roadmap doesn't have to be static, one-off and done. Instead, good product roadmaps are agile and constantly incorporate new customer feedback.