Electromagnetic (EM) radiation is a stream of photons, traveling in waves. The photon is the base particle for all forms of EM radiation
But what's a photon? It's a bundle of energy -- of light -- always in motion. In fact, the amount of energy a photon carries makes it sometimes behave like a wave and sometimes like a particle.
Scientists call this wave-particle duality. Low-energy photons (such as radio) behave like waves, while high-energy photons (such as X-rays) behave more like particles.
EM radiation can travel through empty space. This differentiates it from other types of waves, such as sound, which need a medium to move through.
All forms of EM radiation reside on the electromagnetic spectrum, which ranks radiation from lowest energy/longest wavelength to highest energy/shortest wavelength.
The higher the energy, the stronger, and therefore more dangerous, the radiation.