The main responsibility of the Public Defender's Office is to represent those persons who have been accused of a crime that carries with it the potential for jail or prison time and who cannot afford to retain a private attorney. Only a Judge can appoint a public defender to your case.
A Public Defender can likewise be appointed in juvenile matters of abuse and neglect, or delinquency. A Public Defender could be appointed Guardian Ad Litem of the minor (someone who takes only the best interests of the minor into consideration), or be appointed to represent one or both of the child's parents.