Is There Really THAT Much Difference Between a Project and Product Manager?

In my experience having worked with talented Product Managers and Project Managers, as well as people who seem to think they are interchangeable, I'd define the difference as follows:

[Product Manager]

- Responsible for knowing the market, customer, and product inside and out.

- Defines the product roadmap, helps write specs, etc.

- Success is defined by whether the product is successful in market (or whatever specific goals are of the feature they are working on, etc.)

[Project Manager]

- Primarily responsible for making sure things get done to spec on time.

- Assists with resource allocation for team members to make best use of time, and help keep the roadmap on schedule and on/under budget.

- Not responsible for making product decisions, or the end impact of those decisions. Purely responsible for getting the project out the door successfully in the most efficient and effective manner possible.

So at the end of the day, the Product Manager decides what gets shipped, while the Project Manager makes sure it ships on time and on/under budget in conjunction with everything else.