The judicial system in most common law nations features an adversarial model in the conduct of legal proceedings. This requires the participation of two sides (parties) who argue before the court their opposing versions of the facts and the applicable law. Some key elements of the adversarial model are as follows:
Witnesses are examined orally first by the party who called them and then cross-examined by the other side.
Civil matters - the parties are the plaintiff and defendant
Criminal matters - the parties are the Crown (or State) and the defendant. A prosecutor conducts the case on behalf of the Crown. In the Magistrates Court the prosecutor is a police officer whereas in the higher courts the prosecution is conducted by the Director of Public Prosecutions.
The plaintiff or prosecutor must prove their case before the court.