A police car is a car that holds police officers.

You knew THAT much already, right? Good, now I can get into some more specifics.

Police cars in the United States are typically very large automobiles. Typical models are Ford Crown Victorias and Chevrolet Caprices. Big engines are the norm. The car (officially called a cruiser, unofficially called a black and white in old movies) is stereotypically painted white, but many localities paint theirs red, grey, or something else. The cruiser will be marked with one of several things:

The inside of a cop car is a bit different from your average large sedan. Arrested folks get tossed in the back, which has NO way to open the windows or doors. There are, of course, radios and things (hey, those antennas aren't there for show!), radar guns, and other equipment that obviously depends on whose police car it is!
A police siren backed up by a dance beat was the basis for Speedy J's single 'Pull Over' on Plus 8 Records. It made the Dutch Top 40.
Nodeshell challenge from Gorgonzola