The Internal Affairs Bureau (IAB) is a unit that investigates potential misconduct, either protocol violation or criminal acts, within the New York City Police Department.
One of the main purposes of the bureau is in effect, to "police" the employees of the department. Officers and detectives must always remember to strive constantly to live up to the highest standards of conduct and professionalism in policing. And if that standard is compromised, the Internal Affairs Bureau steps in. Any NYPD officer that discharges his weapon in the line of duty is required to submit to interview by IAB officers in order to determine whether the officer was justified in doing so.
The Internal Affairs Bureau isn't very popular among police officers and detectives. The officers from the Bureau are derogatorily called "rats" or "cheese eaters" by other officers, and IAB itself is nicknamed, in a similar fashion, the "Rat Squad."