Generally, police may ask you to identify yourself or question you briefly without arresting you. They may also issue a citation to you for a summary offense (such as a moving violation or minor non-traffic offense such as public drunkenness). If an officer takes you into custody or otherwise deprives you of your freedom, informs you of your rights or tells that you are under arrest and/or indicates that you are being held for a crime, you have been arrested.
While it is true that you are not required to talk to the police if you are not under arrest, it is a crime to resist arrest by the police. The officer may use reasonable force if necessary to make the arrest. As such, you should not resist an officer arresting you or interfere in the arrest of another person. Once arrested, you should refuse to speak with the police until consulting an attorney. In addition, if you think your rights are being violated, remember exactly what is being done and consult an attorney about it as soon as possible.