The electromagnetic spectrum describes all kinds of electromagnetic wave. All these waves travel at the speed of light. What kind of wave they are is determined by their wavelength (the distance between the peak of each wave). The shorter the wavelength, the more energy the wave will have.
The shortest wavelengths are gamma rays. This is a type of radiation given off by nuclear material. They can be very high energy, so you need to be very careful – you can die quite quickly from excessive gamma ray exposure.
Next shortest are X-rays. These have enough energy to go through you and image your bones. They’re not as dangerous as gamma rays, so having an X-ray won’t kill you. But if you were exposed to lots and lots then it would do some damage. That’s why when you have an X-ray, the nurse will stand behind a screen – it wouldn’t be safe for them to be near the X-rays over and over and over every day.
Next shortest are ultraviolet. We can’t see ultraviolet, but a lot of birds and insects can. Just longer than ultraviolet is visible light. blue light has the shortest wavelength, and red light has the longest. Because they have different wavelengths, they can be diffracted by a prism (or by rain-water droplets), allowing you to see all the different colours in a rainbow.
Just longer than visible light is infrared. Most hot objects radiate a lot of infrared – it’s what we use for those heat-sensitive cameras that you might have seen.
Next longer wavelength after infrared are microwaves. You know these because we use them to cook. Despite this, they don’t actually have enough energy to cause health problems to humans. We also use them for some telecommunications.
The longest electromagnetic waves are radio waves, which again you should be familiar with.
So all forms of electromagnetic radiation are related to each other – the important factor is the wavelength (or frequency): from high frequency, small wavelength and highly energetic and dangerous gamma rays, to low frequency, long wavelength, low energy and completely safe radio waves.