The Product Owner is the empowered central point of product leadership. It is one of the three collaborating roles that constitute every Scrum Team (the others being the Scrum Master and the Development Team).
The Product Owner needs to look in at least two directions simultaneously. On one hand, the Product Owner must understand the needs and priorities of the organizational stakeholders, the customers, and the users well enough to act as their voice. In this respect the Product Owner acts as a product manager, ensuring that the right solution is developed. On the other hand, the product owner must communicate to the Development Team what to build and the order in which to build it. The product owner must also ensure that the criteria for accepting features are specified and the tests that verify those criteria are later run to determine whether the features are complete. The product owner doesn’t write detail-level tests but ensures that the high-level ones are written so that the team can determine when the product owner will consider the feature complete. In these respects the product owner is part business analyst and part tester.
The Product Owner is the single individual who is responsible for drawing out the most valuable possible product by the desired date. This is done by managing the flow of work into the team, selecting and refining items from the Product Backlog. The Product Owner maintains the Product Backlog and ensures everyone knows what is on it and what the priorities are. The Product Owner may be supported by other individuals, but must be a single person.
Certainly the Product Owner is not solely responsible for everything. The whole Scrum Team is responsible for being as productive as possible, for improving their practices, for asking the right questions, for helping the Product Owner, and so on.
The Development Team is responsible for determining how much work will be taken on in a Sprint, and for producing a usable Product Increment in every Sprint. Nonetheless, the Product Owner, in Scrum, is an unique position. The Product Owner is typically the individual closest to the “business side” of the project. The Product Owner is typically charged by the organization to “get this product out”, and is typically the person who is expected to do the best possible job of satisfying all the stakeholders. The Product Owner does this by managing the Product Backlog, and by ensuring the Product Backlog, and progress against it , is kept visible. The Product Owner, by choosing what the Development Team should do next and what to defer, makes the scope versus schedule decisions to lead the best possible product.
The Product Owner has to have the right balance:
Understands thoroughly the needs of the customer YET they understand what it takes to build software
Senior enough to make the decisions YET they are available to the team
Knows when to say no YET they have good working relationship with stakeholders and the team
Understands how to create value YET they trust the team to get the work done
This is clearly a full-time role with significant responsibilities. As a matter of fact, you might think that it is not practical for one person to handle all of these responsibilities or to have all of the attributes necessary to be successful in the role. In most cases a single person can and should fill the product owner role; however, under certain circumstances, product owner teams or product owner proxies might be practical.