The terms "policeman" and "police officer" for the most part are technically interchangeable and mean literally the same thing - a member of a police force. These words could both be used for male or female police personnel. However, it's generally more acceptable to refer to the non-gender version, "police officer" in order to avoid the appearance of bias against women police personnel. Colloquially, "policeman" and "policewoman" are still used, but for most formal reference the term "police officer" is most suitable.

(Note that "police officer" could also refer to a canine (dog) officer! (male or female) :D)