What is radiation, you ask? Radiation in physics is the process of emitting energy in the form of waves or particles. Various types of radiation may be distinguished, depending on the properties of the emitted energy/matter, the type of the emission source, properties and purposes of the emission, etc. When used by the general public, the word "radiation" commonly refers to ionizing radiation.
PHOTO: Uranium Information Center
Dr. Frank Settle states on his web site, Chemcases.com:
The radiation produced by nuclear reactions interacts with living tissue in many ways depending on the type of radiation. This radiation includes high-energy, charged particles (alpha and beta), neutrons of various energies, and photons (gamma and x-rays). In addition to this primary radiation, fission also produces radioactive isotopes of many elements, which in turn can emit particles and photons, known as secondary radiation.
Many of these isotopes, such as strontium and iodine, can enter the body, where they replace non-radioactive elements and remain there emitting ionizing radiation. In many ways, the presence of these radioactive isotopes is more insidious than direct radiation from external sources that is more easily detected and reduced by proper shielding.
Dr. Settle also states:
The ability to penetrate matter differs greatly among the various types of nuclear radiation. A sheet of paper, a layer of clothing, or an inch of air can stop relatively slow moving, heavy alpha particles. Thus, it is easy to shield against alpha radiation, unless the alpha-emitting substance enters the body. Beta particles are lighter and travel faster than alpha particles. They can penetrate a fraction of an inch in solids and liquids and several feet in air.
Gamma rays and neutrons are electrically neutral and thus not slowed by collisions with the target materials. They do not interact strongly as the charged alpha and beta particles do and are therefore highly penetrating. Their ability to penetrate the target material depends upon their energy. High-energy gamma rays may require several feet of material for adequate shielding.
Note: For a more thorough understanding of nuclear radiation, please read Nuclear Weapon Radiation Effects.