Hi drac1,
A private person in a public place can strike up a conversation and ask you questions. A police officer can do the same. It's called a consensual encounter and you have the right to either speak with the officer, or to decide not to do so just like you have the right to choose to whether or not to speak with a private person. Nothing says the officer can't ask to see your identification or ask you questions. However, during a consensual encounter, you are not obligated to provide identification or answer those questions.
There are many reasons for a police officer to strike up a conversation with a member of the public. However, if the officer wishes to detain a person, that is when they must have reasonable suspicion to believe the person is involved with criminal activity. Most vehicle stops and pedestrian stops are detentions and require reasonable suspicion. However, it does not usually take much for a good police officer to develop reasonable suspicion to stop someone. If you are not sure if you are being detained or not, simply ask. Be civil about it and there should be no issues. Remember, the police are just trying to do their jobs. They should be professional and courteous, (in the absence of extenuating circumstances) and mutual respect and cooperation will go along way toward a positive result.