It just depends on the energy - the wavelength. Our eyes just happen to be sensitive to the part of the spectrum we call "light", so we called it light... You can "see" by just about any type of radiation using the proper camera.
Yes, radio waves only by virtue of their "lower than light"...
Electromagnetism is a property of current flowing, or moving electrons. When electrons emit photons you get a electromagnetic "wave", which may be a radio wave or a light wave.
The electromagnetic spectrum is a mapping of electromagnetic waves, from the longest wave to the shortest wave...