In Spain, a denuncia is the report of a crime to the police, a prosecutor or a judge. The alleged offense is then investigated and, if found to have merit, prosecuted. But once a denuncia is made, it is out of the hands of the informant who reported the crime.
In contrast, in a querella the person reporting the crime becomes a party to the proceedings. This is often the public prosecutor, in which case this is known as a querella del Ministerio Fiscal (“criminal complaint filed by the Public Prosecution Service”). But in contrast to Anglo-American criminal procedure, in Spain the victim or, at times, even an ordinary citizen may report a crime and indicate that he intends to enter an appearance as a party to the criminal proceedings, and his lawyer then will prosecute the case alongside the public prosecutor (fiscal).
In that regard, in addition to the powers of the public prosecutor to prosecute criminal offenses on behalf of the state, in Spain the victim or any other private citizen may enter an appearance (personarse) in criminal proceedings in what amounts to a “private prosecution” that is practically unknown or in disuse in Anglo-American jurisdictions. In this context “acusación (or) acusador particular” generally denotes the victim of a crime (or his representative) who files a private criminal complaint, entering an appearance in a criminal proceeding as a private prosecutor. Moreover, persons other than the victim may also enter an appearance in criminal proceedings as private prosecutors if they believe that they have an interest to defend in the matter. In that regard acusación (or) acusador popular denotes a private individual or an association of citizens who file a querella and post a bond (fianza) in order to be admitted as a party to the prosecution of a criminal case. And acusación (or) acusador privado refers to an individual seeking redress for a private offense (delito privado) that may only be prosecuted by the victim and in which the public prosecutor does not intervene.
In summary, denuncia is a “crime report” or the “report of a crime to the police (or other authority),” while in the cases initiated by private individuals querella may perhaps be described as a “private criminal prosecution.” For more on this peculiar Spanish system of allowing private individuals to participate as parties to criminal prosecutions see:
Julio Pérez Gil. “Private Interests Seeking Punishment: Prosecution Brought by Private Individuals and Groups in Spain.” Law & Policy, Volume 25 Issue 2 (April, 2003), pp. 151-172.