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Electromagnetic radiation includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. All of these types of radiation can be thought of as waves, like the ripples that spread out when you drop a pebble into a calm pool of water. An electromagnetic wave has a few important properties:

Speed: how fast is each ripple moving?
Frequency: if you point at the water with your finger, how many ripples pass by your finger every second?
Wavelength: the distance between two adjacent ripples.
The speed is easy. It turns out that all electromagnetic waves have the same speed, which scientists represent with the letter c. This speed, the speed of light, is equal to 670 million miles per hour.

The frequency can be any number. It is measured in Hertz, which means "One ripple per second." If two ripples pass by your finger every second, that's 2 Hertz. Most electromagnetic radiation has frequencies much larger than 1 Hertz.

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