Radiation with sufficiently high energy can ionize atoms; that is to say it can knock electrons off of atoms and create ions. This occurs when an electron is stripped (or "knocked out") from an electron shell of the atom, which leaves the atom with a net positive charge. Because living cells and, more importantly, the DNA in those cells can be damaged by this ionization, it can result in an increased chance of cancer.
Thus "ionizing radiation" is somewhat artificially separated from particle radiation and electromagnetic radiation, simply due to its great potential for biological damage. While an individual cell is made of trillions of atoms, only a small fraction of those will be ionized at low radiation powers.
The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, and is a function of the damaging tendency of the type of radiation (equivalent dose) and the sensitivity of the irradiated organism or tissues (effective dose).
Ionizing radiation originates from radioactive materials, X-ray tubes, particle accelerators, and is naturally present in the environment. It is invisible and not directly detectable by human senses; as a result, instruments such as Geiger counters are usually required to detect its presence. In some cases, it may lead to secondary emission of visible light upon its interaction with matter, as in the case of Cherenkov radiation and radio-luminescence.
Ionizing radiation has many practical uses in medicine, research and construction, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue; high doses result in skin burns, radiation sickness and death, while low but persistent doses result in cancer tumors and genetic damage.
Electromagnetic radiation (EMR) is represented as self-propagating waves. EMR has electric and magnetic field components that oscillate in phase perpendicular to each other and also to the direction of energy propagation. EMR is classified into types according to the frequency range of the waves, these types include (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Of these, radio waves have the longest wavelengths (lowest energy) and gamma rays have the shortest and hence the highest energy. A small window of frequencies, called the visible spectrum or light, is sensed by the eyes of various organisms.
Ionizing electromagnetic radiation is that for which the photons making up the radiation have energies larger than about 10 electron volts. The ability of an electromagnetic wave (photons) to ionize an atom or molecule thus depends on its frequency, which determines the energy of a photon of the radiation.
An energy of 10 eV is about 1.6×10 joules, which is a typical binding energy of an outer electron to an atom or organic molecule. This corresponds with a frequency of 2.4×1015 Hz, and a wavelength of 125 nm (this is in far ultraviolet) or less.(wikipedia.org)