Scott Colfer writes about product roadmaps in this article, but it includes what I think is a pretty valuable overview of the core role of a product manager:

Product management makes promises to solve problems over commitments to specific solutions. We take the human tendency to specify the means of doing something, rather than the result that is wanted, and redirect it by specifying what is to be achieved, rather than how to achieve it. Product managers are usefully agnostic about solutions — we help teams to prioritize the most valuable problems to solve and then help them to prioritize the solution that requires the least work for the most value — and we’re happy when we learn something that helps us to refine and simplify our solution.