It's pretty much in the difference between a project and a product: by most definitions, a project is a tool for change in an organization, and has a limited lifetime and a limited scope. A product, on the other hand, is a tool for performing some business function (either internally for staff or externally through sales), and its scope can be whatever the business needs.
Typically, a project manager has authority over resource allocation and resource planning, but must defer to the stakeholders for everything else (most importantly, for actual design/implementation decisions). A product manager has authority over the complete lifecycle of his product, including design, implementation and resource allocation.
Typically, a project manager has authority over resource allocation and resource planning, but must defer to the stakeholders for everything else (most importantly, for actual design/implementation decisions). A product manager has authority over the complete lifecycle of his product, including design, implementation and resource allocation.