Step 3: Resistance
For instance, in the circuit above, the motor that electricity is flowing through is adding resistance to the flow of electricity. Thus, all of the electricity passing through the circuit is being put to use.
In other words, there needs to be something wired between positive and ground that adds resistance to the flow of electricity and uses it up. If positive voltage is connected directly to ground and does not first pass through something that adds resistance, like a motor, this will result in a short circuit. This means that the positive voltage is connected directly to ground.
Likewise, if electricity passes through a component (or group of components) that does not add enough resistance to the circuit, a short will likewise occur (see Ohm's Law video).
Shorts are bad because they will result in your battery and/or circuit overheating, breaking, catching on fire, and/or exploding.
It is very important to prevent short circuits by making sure that the positive voltage is never wired directly to ground.
That said, always keep in mind that electricity always follows the path of least resistance to ground. What this means is that if you give positive voltage the choice to pass through a motor to ground, or follow a wire straight to ground, it will follow the wire because the wire provides the least resistance. This also means that by using the wire to bypass the source of resistance straight to ground, you have created a short circuit. Always make sure that you never accidentally connect positive voltage to ground while wiring things in parallel.
Also note that a switch does not add any resistance to a circuit and simply adding a switch between power and ground will create a short circuit.