Answer:
Nothing, because they are due to the same exact event. Thunder is what you hear, lightning is what you see, but both are due to electrical discharge in the atmosphere. They can seem to be separated in time to an observer (you see the lightening before you hear the thunder) because of the large difference in the speed of sound and the speed of light. If you are significant distance away (a few miles or kilometers), the light from the discharge will reach you almost instantly, but the sound can take several seconds to arrive. In fact you can measure the distance from you to the lightening in this way: for each second delay between seeing it and hearing it, the lightening bolt is approximately 300 meters, or 1000 ft away.
Thunder is merely the rumbling sound that lightning creates. You hear the thunder after you see the lighting because light travels faster than sound.
Lightning is the light that is emitted, and thunder is the sound.