Electromagnetic waves consist of travelling electric and magnetic fields. Unlike water waves or sound waves, electromagnetic waves can travel and transmit energy in free space — that is, they don’t need a medium to travel. Electromagnetic waves can travel very (very) fast: 300 million metres per second, or more than 670 million miles per hour in a vacuum.
The waves in the electromagnetic spectrum are identified by their wavelength (a measure of the distance between adjacent peaks and troughs) and frequency (the number of cycles of a wave passing a point in one second). You can learn about how wavelength, frequency and energy inter-relate in this fun activity.
Electromagnetic waves occupy a continuous spectrum. At the low-frequency, long wavelength end of the scale are radio waves. At the other extreme are very high energy, high frequency gamma rays. In between are microwaves, light, and X-rays. Visible light — arguably the most important part of the electromagnetic spectrum for us humans — is a small part of the full optical spectrum which also includes infrared and ultraviolet light.
You can find out more from this introductory video, brought to you by NASA Science.