A capacitor is a component that stores electrical energy. It's like a low capacity, fast charging battery—except a battery stores energy via an electrochemical process, whereas capacitors store energy via electrical fields.

Capacitors are often used to store bursts of energy and filter/smooth out voltage fluctuations. Most circuit boards will have tiny surface-mount capacitors, while power supplies will have bigger cylindrical ones.

Thumb-sized capacitors can store enough energy to pose a risk of electric shock. Use a capacitor discharge tool to safely discharge larger capacitors.

Capacitance is a measurement of how much a capacitor can store at a given voltage. The SI unit of measurement for capacitance is a Farad (F). Most capacitors have a nF (nanofarad) or μF (microfarad) capacity.

Larger capacitors such as electrolytic capacitors are polarized, meaning they should only be connected in a specific direction. If they're used in reverse for extended periods of time, they will fail and often burst.

When a capacitor fails, it usually short-circuits. The dielectric material which insulates the layers breaks down and the layers conduct, shorting the capacitor. A working capacitor should never have a resistance measurement of less than a few kilohms.

The multimeter measures capacitance by charging the capacitor with constant current, recording how long it takes for the capacitor to charge/discharge, and calculating the capacitance value with an equation. Depending on the size of the capacitor, the multimeter may not give a very accurate reading.

You'll often need to decouple the capacitor from the circuit in order to measure it. Capacitors are commonly integrated in parallel to other components in a circuit, making direct measurement difficult. To decouple the capacitor, desolder/disconnect one end of the capacitor from the circuit.