What is an EM Wave?

When an electromagnetic wave travels from A to B what is actually moving (apart from energy)?

Imagine space to contain electric E and magnetic B fields. These fields are usually at zero like the flat surface of a still lake, but they can be disturbed by the movement of a charged object, e.g. a spark. This creates a disturbance in these fields which travels away from the disturbance at the speed of light. The second picture below shows Marconi and his spark transmitter.

An electromagnetic wave is therefore an oscillating E field and B field, both of which are perpendicular to each other and to the direction of propagation of the wave. In the 1880's James Clarke Maxwell and a few other derived a set of equations which describe the nature of EM waves in terms of oscillating E and B fields. If you study Physics beyond A level you will certainly come across Maxwell's Equations.