EM AND GRAVITY

This thread is about the relationship between electromagnetism and gravity.  (first, a small description of electromagnetism and gravity would be enjoyable.) second, how would one define that relationship?  does EM react to gravity, or gravity react to EM? or is it both?  would it theoretically be possible to use a for of "coil" to produce a gravitational force?  how would one go about doing that?  to be brief, any information about this would be very appreciated.