The Police Cruiser is one of two Police Cars used by the LCPD in GTA IV, alongside the Police Patrol. It can be found at almost any LCPD station, and is dressed in LCPD livery (based on that of the NYPD).
The Cruiser appears to be modeled after a Ford Crown Victoria Police Interceptor. It's taillights are similar to those of a 1992-1994 model; the windows, headlights, and front and rear bumpers resemble those of the 1992-97 models; the chassis (on the bottom) is from 1995-1997 models; and the grille and hood look like those of 1998 and later models. The dashboard instrument cluster is unlike the Ford product, consisting of a tachometer, speedometer, fuel gauge and temperature gauge rather than the Crown Victoria's central speedometer and gauges for fuel, temperature, oil pressure and volts.
According to in-game data files, the NOOSE/Police Cruiser car is the 9th/10th fastest car in the game, with a top speed of 194mph.
When you are stationary in the Police Cruiser you can access the Police Computer (this is done by pressing the L1 button). Which lets you search for the most wanted criminals, see what the most recent crimes are, search the police records and even call for backup. This is where you can start the 'Vigilante' style side missions and the 'Most Wanted' side missions.