A cathode refers to the more negative lead of a polarized component such as an LED. Components with cathodes also have anodes, the more positive lead.
Typically, the longer lead on a polarized component is the anode and the shorter lead is the cathode. Of course, that's if someone hasn't trimmed the leads.
To help avoid polarity confusion, manufacturers add other small details to components to help you determine if a lead is more positive or negative. For example:
Look closely at an LED from the top and you'll notice that one of the sides is flat, which indicates the side with the more negative lead.
Electrolytic Capacitors retain their cylindrical shape, and instead add a fancy painted stripe on their side to indicate their more negative lead.