What is a wave?
In general, we can define a wave as a disturbance moving in a given environment, without transport of matter with this wave. This disturbance produces on its way a reversible variation of local physical properties. The wave is a way of representing a physical phenomenon, and thus applies to very different fields: include mechanical waves (earthquakes, spring, waves), sound waves, shock waves, the electromagnetic waves ...
For example, movements that agitate a stretched rope are also waves: if you give one tip of the rope a jerk, you will see a distortion of the rope (the disturbance), move to the other tip. It is once again a disturbance that moves, that is called a wave.
Similarly, to quote another classic example, if you place a paper boat in a pond and you throw a stone nearby, it will create a wave, propagating the disturbance of the surface water. Throwing the stone, the height of the surface water has changed. This change of the height is conveyed around, until arriving at the paper boat, which is lifted from the bottom up, but that does not move on the surface. The water moves only vertically, not horizontally. This example also allows us to demonstrate that the material in which the wave propagates does not move.
Similarly, an electromagnetic wave only changes the characteristics of the electromagnetic field, making them oscillate around their initial value. An electromagnetic wave can propagate in both vacuum and material media. The speed of an electromagnetic wave depends on the environment it traverses. Light is the most obvious and best known of electromagnetic waves.
Light travels through space at a speed of about 300 000 km / s, it takes for instance 8 minutes and 20 seconds to reach us from the sun.