A very common term in electronics is the word circuit. In general, you will hear the word used to describe what a schematic shows. Most projects are a circuit and most schematics are a circuit.
A Circuit is a Circle
The literal definition of a circuit comes from "circle", and is used to refer to the path that electricity flows in a project. Let's look at a simple circuit.
Above is a circuit consisting of a battery and a lightbulb. Notice how it makes a circle. The circuit is complete, so the lightbulb is lit. This is called a closed circuit.
If you do not have a closed circuit, then you have an open circuit. Let's look at some open circuits.
Open Circuit Example 1: missing top wire.
In Example 1 we have left off the top wire. This is an open circuit. Notice that the light bulb is off. Without a complete circuit, the light bulb will not be lit. In this example the light bulb is said to be missing voltage, because ground is hooked up but the wire providing +9 volts is not.
Open Circuit Example 2: missing bottom wire.
In Example 2 we have left off the bottom wire. This is also an open circuit and the light bulb is off. In this example, the light bulb is said to be missing ground because the light bulb has +9 volts but doesn't have a ground connection.
Note that these are still circuits, they just happen to be open circuits.
Switches in Circuits
Switches are a way of manually opening or closing a circuit. Switches come in many varieties, including push button, slide and toggle.
The buttons on a telephone keypad and the buttons on your remote are also switches.
Switch Example 1: Open Switch = Open Circuit
In Example 1 the switch is open, so the circuit is open and the light bulb is off. Opening a switch has the same effect on a circuit as disconnecting a wire.
Switch Example 2: Closed Switch = Closed Circuit
In Example 2 the switch is closed, so the circuit is closed and the light bulb is on. Closing a switch has the same effect on a circuit as connecting a wire.
Not All Circuits Look Complete
In a previous guide we talked about using V+ and GND instead of a battery symbol in a schematic. Keep in mind that when you do that, your closed circuits will no longer make circles, but they are still closed circuits. Let's take a look at an example.
Above is a circuit consisting of a battery and a lightbulb. See how the schematic doesn't make a circle any more, but the real life example does? This is still a closed circuit and the light bulb is still lit.
All About Circuits
In general, you can tell if you have a closed circuit using the following rule:
If the circuit works then it's a closed circuit.
If you are using a motor, and the motor is turning then the circuit is closed. If you are using a buzzer and the buzzer is buzzing then your circuit is closed. When using a light bulb, the glowing light means you have a closed circuit.
In all of these cases the part in question (motor, buzzer, light bulb) is doing what it is supposed to do, so your circuit is closed.