Fuel (the energy source, usually hydrogen) and air (which contains the oxygen) are put into opposite sides the fuel cell. In the middle of the fuel cell there is a "screen", called an electrolyte, sandwiched between two metal plates, called electrodes, which keeps the fuel and air separate. Different kinds of fuel cells get their names based on what type of screen is used to separate the fuel and air. The screen only allows specific charged molecules, also known as ions, to pass through them.