James Shore, recipient of the 2005 Gordon Pask Award for Contributions to Agile Practice, write a blog called Successful Software.
In a recent blog posting, he wrote “To recap, the product manager is the person on the team who is responsible for maintaining the vision of the product the team is producing. He (or she) must have a strong sense of what value the product brings to the organization or its customers and he has to have the authority and political know-how to tie together many disparate interests and desires. This person also needs to be easily available to the team to make the tough calls about priorities, what goes in to the product, and what stays out. On an agile team, he should also participate in every planning meeting and review every demo.”
The rest of the blog post can be found at this link.