Generally speaking, a Product Manager is responsible for defining the features that are to be built, and not necessarily the details of the construction or implementation of those features. They will work with the Project Manager to ensure the requirements are understood, and the Project Manager develops and executes on a plan to create the new features. Depending on what your development team looks like, or the SDLC methodology you are using, the lines between Product Manager and Project Manager can get a little blurry. If you look at it as the ProdMan is responsible for the vision and high-level strategy and the ProjMan is responsible for the tactical planning and execution, that's probably a pretty good place to start creating definitions around the different roles.