Radiation is a general term that includes light, sound and radio waves. All three transmit some kind of energy through space. But ionizing radiation can also disrupt the atoms and molecules within the human body.
Atoms are made of protons and neutrons contained in a nucleus, and electrons that orbit the nucleus. The number of protons usually is the same as the number of neutrons. Ions are atoms that don't have the same number of protons and electrons. Ionizing radiation can knock electrons out of atoms to create ions. The resulting free electrons then collide with other atoms to create even more ions.
This is dangerous because an ion's electrical charge can lead to unnatural chemical reactions inside cells, particularly at the higher energy levels of X-rays and gamma rays. It can break DNA chains, causing the cell to either die or develop a mutation and become cancerous, which can then spread. And if the mutation occurs in a sperm or egg, the result can be birth defects, which is why pregnant women should never be subjected to X-rays.