It depends a lot on the company, but:<p>1. Project manager: make sure the product (as defined by someone else) ships on schedule with the top priority features (as defined by someone else).<p>2. Product manager: make sure the product meets user needs. This may involve some project management as well.
A project has a shorted life-span and specific outputs that would mark the end of the project.<p>A product might evolve and run "forever" without a specific outputs to mark the end of the product.