Here is Tony Fadell's description of a product manager in Build: An Unorthodox Guide to Making Things Worth Making:
Product Manager or Product Marketing Manager
Product marketing and product management are essentially the same thing--or at least they should be. A product manager's responsibility is to figure out what the product should do and then create the spec (the description of how it will work) as well as the messaging (the facts you want customers to understand). Then they work with almost every part of the business (engineering, design, customer support, finance, sales, marketing, etc.) to get the product spec'd, built, and brought to market. They ensure that it stays true to its original intent and doesn't get watered down along the way. But, most importantly, product managers are the voice of the customer. They keep every team in check to make sure they don't lose sight of the ultimate goal—happy, satisfied customers.
The customer needs a voice on the team. Engineers like to build products using the coolest new technology. Sales wants to build products that will make them a lot of money. But the product manager's sole focus and responsibility is to build the right products for their customers. That's the job.
The tricky thing is that the responsibilities of a product manager are completely different at different companies. Product management is less a well-defined role and more a set of skills. It lives between everything, a white space that morphs based on the customer, the needs of the business, and the abilities of the humans involved.
A good product manager will do a little of everything and a great deal of all this:
- Spec out what the product should do and the road map for where it will go over time.
- Determine and maintain the messaging matrix.
- Work with engineering to get the product built according to spec.
- Work with design to make it intuitive and attractive to the target customer.
- Work with marketing to help them understand the technical nuances in order to develop effective creative to communicate the messaging.
- Present the product to management and get feedback from the execs.
- Work with sales and finance to make sure this product has a market and can eventually make money.
- Work with customer support to write necessary instructions, help manage problems, and take in customer requests and complaints.
- Work with PR to address public perceptions, write the mock press release, and often act as a spokesperson.
And then there's the even less well-defined stuff. Product managers look for places where the customer is unhappy. They unravel issues as they go, discovering the root of the problem and working with the team to solve it. They do whatever is necessary to move projects forward that could be taking notes in meetings or triaging bugs or summarizing customer feedback or organizing team docs or sitting down with designers and sketching something out or meeting with engineering and digging into the code. It's different for every product.