Why is light the same thing as electromagnetic radiation?
All electromagnetic radiation is in the form of photons and could then therefore said to be light.
Any body that possesses heat can produce radiation. Depending on the electromagnetic processes that are ongoing in that body will determine how that radiation is released.
Electromagnetic energy travels in the form of waves. The wavelength will determine what form that energy takes. Visible light is only a small part of the spectrum. The shortest wavelengths are things like x-rays and gamma rays.
That all being said all the different radiations in the electromagnetic spectrum are all photons albeit photons of extremely different energy levels. Shorter wave length particles have higher energy.