There's a difference between an "Unmarked Cruiser" and an "Undercover Car".
Typically, an Unmarked Cruiser is the same make and model as the standard police cruisers, including all the engine beef-ups. They simply lack the paint job and markngs that identify it as a police vehicle. An undercover car, on the other hand, is usually Just Another Car(tm), and is not made to draw attention to itself. Picture something like an old, rusted out Toyota Camry.
Unmarked Cruisers are most commonly used for standard patrol, while undercover cars are usually for sting operations, and/or stakeouts.
My first speeding ticket was courtesy of a police officer in an Unmarked Cruiser. It was a light brown Ford Crown Victoria.