Applet that shows discrete emission and absorption spectra of the elements (University of Oregon).
Electromagnetic waves are waves made of changing electric and magnetic fields. The electric wave part is perpendicular to the magnetic wave part and in phase with it. In a vacuum all types of electromagnetic waves travel at the same speed, c = 2.99 × 108 m/s. The difference between, say gamma-rays and visible light (aside from the fact that we can see visible but not gamma-rays) is the frequency and energy. Since frequency and wavelength are related (c = λ f) this also means they have different wavelengths.
The energy of an electromagnetic wave arrives in lumps called photons. The energy of an individual photon for a wave with frequency f is given by E = hf where h = 6.63 × 10-34 J s is Planck’s constant. Waves with frequencies below that of light (for example cell phone signals) have photons that don’t carry very much energy and so are not harmful (do not cause cancer). This is referred to as non-ionizing radiation. On the other hand, the photons of electromagnetic waves such as x-rays and gamma-rays each carry a lot of energy and are very harmful. For these waves each photon carries a lot of energy which can damage the chemical reactions inside cells in a living organism. Electromagnetic waves with frequencies higher than light are said to be ionizing radiation.
Another chart of the electromagnetic spectra.