A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit.
Potential Difference: This is the work done per unit charge. It's what the voltmeter measures.
Circuit: This is a closed path that allows electricity to flow from one point to another. It's where you'd use your voltmeter.
Ammeter: This is another type of meter used to measure current in a circuit. It's like the scale to your ruler (voltmeter).