When a person is exposed to ionizing radiation, induced biological phenomena can cause more or less important damage to organism cells. The seriousness of this damage depends on the type of the ionizing radiation, its energy and the sensitivity of the exposed tissues and organs.
Simplified, there are actually two ways in which a human being can be exposed to radioactive radiation: by contamination or by irradiation.
We speak of contamination when there is physical contact with the radioactive particles. That contact might be internal or external. Internal means that the particles have penetrated our organism through respiration or via our digestive system. External means that the particles stick to the surface, e.g. to skin or hair.
In all other cases we speak of irradiation: the radioactive particles are situated outside the organism and emit rays.
Radioactivity is widely used however for the benefit of man (nuclear medicine, sterilization of medical/surgical instruments, protection and conservation of works of art, leak and smoke detectors,...).