(N/A) When Maxwell predicted the existence of electromagnetic waves,only visible light waves were considered electromagnetic. There was no information regarding infrared and ultraviolet rays.
At the end of the $19^{th}$ century,$X$-rays and gamma rays were discovered.
At present,light waves,$X$-rays,gamma rays,radio waves,microwaves,ultraviolet waves,and infrared waves are all part of the electromagnetic spectrum.
When electromagnetic waves are classified based on frequency (or wavelength),it is called the electromagnetic spectrum. In this spectrum,the values of wavelengths are continuously distributed. There is no distinct boundary separating different types of waves.
This classification is based on how these waves are produced or how they can be detected.