You need a [u]Driver Circuit[/u] to control a motor.
The Arduino I/O pins are rated for 40 milliamps maximum. They are "signals", not "power", and about the only thing they can power is an LED. If you try to power a motor you won't get 5V out of the pin (but don't try it because you can potentially damage your Arduino).
In the "famous water analogy" voltage is water pressure and current (Amps or milliamps) is water flow... Resistance is like a small pipe which limits water flow.
The difference is, if you cut a pipe you get low resistance and water flows-out everywhere (but nothing else bad happens with low water-resistance). If you cut a wire you get infinite resistance and no current flows.
And, if electrical resistance is too low you get too much current and things burn-up.
...We generally don't care about the motor's resistance, we just check it's current (and voltage) rating.
[u]Ohm's Law[/u] describes the relationship between voltage, resistance, and current.