If they draw less than 50 mA you can drive it directly from the arduino board with no added circuitry
You know that tiny motor that just about manages to make your mobile phone vibrate?
That would almost certainly overload an AVR pin ( the limit is 40mA, not 50mA) - I've measured some over 130mA, though 70 to 100 is more likely.
Now imagine that vibra motor moving any kind of car bigger than a Hot Wheels.
Use a transistor.