1. Complaint letter: Complaint letter mainly refers to the materials that organs, organizations, enterprises, institutions and citizens report and expose criminal acts to public security organs, people's procuratorates or people's courts according to law, and request that criminals be punished according to law.

2. Filing a case: Filing a case is mainly a decision-making legal document made and used by public security organs or people's procuratorates when they discover criminal facts or criminal suspects and decide to file a case for investigation.

Second, the role is different.

1. Complaint letter: The function of complaint letter is mainly for organs, organizations, enterprises, institutions and citizens to request that criminals be punished according to law.

2. Filing a case: The purpose of filing a case is to show that the public security organ has filed a case and the case has entered the investigation stage.

Third, the complaint organs are different.

1. Appeal: The appeal organ of appeal is the public security organ, the people's procuratorate or the people's court, while the appeal organ of criminal private prosecution can only be the people's court.

2. Letter of filing a case: Letter of filing a case means that the people's court, people's procuratorate or public security organ shall promptly review the materials reported, accused, reported or surrendered according to their jurisdiction. If it is believed that there are criminal facts that need to be investigated for criminal responsibility, it shall be filed.

Legal basis: Criminal Procedure Law of People's Republic of China (PRC).

Article 109 When a public security organ or a people's procuratorate discovers a criminal fact or a criminal suspect, it shall file a case for investigation within its jurisdiction.

Article 110 Any unit or individual who discovers criminal facts or suspects has the right and obligation to report to the public security organ, the people's procuratorate or the people's court.

Victims have the right to report and accuse criminal facts or criminal suspects who infringe their personal and property rights to public security organs, people's procuratorates or people's courts.

Public security organs, people's procuratorates or people's courts shall accept reports, complaints and reports. If it does not belong to its own jurisdiction, it shall be transferred to the competent authority for handling, and the informant, complainant and informant shall be notified; For those who are not under their jurisdiction and must take emergency measures, they should take emergency measures first and then transfer them to the competent department.

Where a criminal surrenders himself to a public security organ, a people's procuratorate or a people's court, the provisions of the third paragraph shall apply.

Article 111 Reports, complaints and reports may be made in writing or orally. The staff who accept the oral report, complaint or report shall make a written record, which shall be signed or sealed by the informant, complainant and informant after being correctly read out.

The staff who accept the complaint and report shall explain the legal responsibility of false accusation and frame-up to the complainant and informant. However, as long as it is not fabricating facts and evidence, even if the facts of the accusation and report are different, or even false accusation, it should be strictly distinguished from false accusation.

Public security organs, people's procuratorates or people's courts shall ensure the safety of informants, complainants, informants and their close relatives. Informants, complainants and informants who are unwilling to disclose their names and report, accuse and report their acts shall keep them confidential.