Capacitors can be charged up and store electricity, similar to a car battery. This can be a hazard if they are charged up to high voltages. If it is necessary, capacitors with large charges should be discharged via a resistor to limit the discharge current. DC current cannot flow through a capacitor since the dielectric forms an open circuit. Capacitors come in all shapes and sizes and are usually marked with their value.
Values are measure in Farads.
Values in Farads are unusual. Most capacitor values are measured in microfarads, nanofarads or picofarads. See the page on Value multipliers to find out more about this. They are often marked with their maximum working voltage. The voltage across the terminals must not exceed this value.