A Product Marketing Manager (PMM) is the person responsible for advertising the value of the product to the marketplace. When compared to a product manager a product marketing manager is very much consumer and market-focused.
A product marketing manager assists the product team and the company by assessing competitor products and conducting extensive research to see how the product is faring in the market.
They are also relied upon for communication strategies, all of the communication that centres around the product and how it is positioned in the market go through a product marketing manager.
If you have experience working at a small startup you understand that in a startup people wear multiple hats. In a startup, or a less differentiated organization, these two roles may overlap; a product manager will shoulder the responsibilities of a product manager and a product marketing manager.
Irrespective of how the roles are staffed in an organization both roles are integral to the success of a product.