A "police procedural" is a work of crime fiction that emphasizes the investigative procedure of a police officer or police department as the protagonist(s). This is in contrasted to other genres that focus on private detectives, amateur investigators or the villains.
While many police procedurals conceal the criminal's identity until the crime is solved in a narrative climax (the so-called "whodunit"), others reveal the perpetrator's identity to the audience early in the narrative, making it an inverted detective story.
Whatever the plot style, the defining element of a police procedural is the attempt to accurately depict the profession of law enforcement, including such police-related topics as forensic science, autopsies, gathering evidence, search warrants, interrogation and adherence to legal restrictions and procedures.
The roots of the police procedural can be traced back to at least the mid-1880s. Perhaps the earliest example of the genre is Wilkie Collins's novel "The Moonstone" (1868), a tale of a Scotland Yard detective investigating the theft of a valuable diamond. A somewthat unusual early example of police procedurals is the "Inspector Bonaparte" Series of Arthur W. Upfield. Bonaparte investigated crime in the Australian Outback.
More recent authors of police procedurals include - among many others - P.D. James (Adam Dalgliesh), Louise Penny (Chief Inspector Gamache), Henning Mankell (Wallander), James Patterson (Alex Cross) or Yrsa Sigurdardottir (Police Officer Huldar). More on Wikipedia