Electricity always comes in two forms of energy - an electric field and a magnetic field and they always travel together in the form of 'waves'.
An 'electro-magnetic' device makes use of *both* types of energy at the same time.
For example a radio works by tuning itself to a specific 'electro-magnetic' wave. This wave happens to also carry a radio programme that the radio plays for you. If you tune it to another frequency you either get a 'hiss' i.e. no information on that electro-magnetic wave or you may get another radio program.
A hard disk is also an electro-magnetic device.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Electro-Magnetic