There is too much confusion when it comes to understanding the roles of product manager and project manager. They sound the same after all but are totally different job functions. 

The product manager is responsible for every stage of a product like development, sales and marketing and customer support after the sale.

He or she has P&L responsibility for the product and is measured by how much income is generated by the product.

A product could have many projects working toward it competition or improvements. As such one or more project managers could report to the product manager.

The project manager is responsible for a given project. At the end of that project, the manager could move to another project related to the product or to a totally a new product.

The product manager stays with the product long after a project is done. 

Below I have listed the responsibilities of each job.

Product manager

• Responsible for every phase and stages of a product lifecycle
• Has ownership of the product in the corporation
• Helps to create the product specification and roadmap  
• Controls the specification for the product, the budget, and the resources      
• Has P&L responsibility
• Directly interfaces with the customer
• Holds the long-term vision of the product  
• Has one or more project managers working on the product
• Reports  to senior management or CEO
• Forecasting responsibility  
• Has responsibility for Sales and marketing groups
• Measured by the success of the product and revenue added to the company
• Stays with the product when the project is completed   

Project Manager     

• Responsible for the project
• Helps to write the specification for the project  
• Reports to the product manager
• Manages budget and resources allocated to the project
• Creates and leads a team, monitors and reports their activities
• Chooses the tools and the project methodology used in the project like the Waterfall, Agile or Hybrid.  
• Measured by on time and on budget delivery of the project
• At the end of the project moves to another project either related to the same product or another  

Conclusion

A product manager acts more like the CEO of a product, or its mother. A project manager helps to bring one or more version of the product to the world, or a midwife.

A good product manager knows the value a good project manager brings to the product and strives to hire the best.

After all, it is the project manager who is ultimately responsible to deliver the product to the market on time and on budget.

But this is the product manager who dreams about the product, natures the idea from concept to successful market introduction. 

The product manager makes sure the product gets required attention by the customer, by the press, and by the senior management.