The Product owner is the person setting the agenda for a Scrum team. There is only one Product owner for each Scrum team, but a person can act as Product owner for multiple teams at the same time.
A good Product owner is in constant tension with his team. He should push the team to its limits and make sure value comes first. He should also continue to challenge the team for more quality and make sure that done is done. He is the single point of entry for new requests.
Naturally a Product owner wants as much as possible as quickly as possible in the best possible quality. It is up to the team to limit this to what they can deliver.
For details about the roles in the various meetings please see the How-to for Product owners
The Product owner maintains and prioritizes the stories in the Product backlog He digests input from sales, partners, market and company strategy and derives goals from it, making sure the most valuable features have the highest priority He then sets Acceptance criteria that work results can be measured against
The Product owner is having full and sole authority over the product backlog. He decides what goes in and at what priority. Therefore he shields all external complexities from the team and the team directs any incoming requests to him for prioritization.
As the Product owner sets the goals at the beginning of each Sprint, he also is the one judging their success in the Review meeting at the end of each Sprint. Balanced requirements and acceptance criteria make sure that this power is not used arbitrarily but following clear objectives.
The Product owner does not have the authority to change the current ongoing sprint, although he can kindly ask the team to consider new items or abandon old ones. If there is a case where the Product owner actually has to intervene in the sprint, the only option he has is to abort the sprint and start over with a new planning meeting. (see the How-to for Product owners)
Involvement in the Scrum of Scrums
The Product owners of all teams participate in the weekly Scrum-of-Scrums meetings for Product owners. There they discuss their high-level-items on the Global Backlog. This is a kind of cross-team product backlog holding the priorities on a high level. They are being tracked weekly and are being prioritized in discussion with Openbravo's Chief Technology Officer during this meeting.