A capacitor is a component that is able to store charge. There are a number of different types of capacitors that have different physical properties, but the two most common that you will encounter are the ceramic capacitor and the electrolytic capacitor.
An electrolytic capacitor is polarized component with a tubular body and two leads. A stripe down the side of the component's body indicates the cathode — the more negative lead.
A ceramic capacitors are disk shaped and are not polarized like an electrolytic capacitor. This means you don't need to worry about which way the ceramic capacitor is wired in a circuit.