Essentially, fleet vehicles are purchased in bulk by some sort of agency, used by that agency for a period of time, then cycled out into used-car lots.
When you see a fleet car while used-car shopping, it’s a good idea to look into its mileage and origin.
Urban fleet cars—like former taxis for example—tend to endure more stop-and-go traffic, which can wear on the engine, transmission, brakes, and suspension. Former rentals, on the other hand, are fairly new vehicles, and most rental drivers try to drive carefully so they don’t get fined.