Radiation in the most basic sense is the act of a material releasing energy. Think of a fire, like one in a fireplace, and how you can feel the heat coming off the flames even though you are standing some distance from the fire. That heat you feel is reaching you by radiation. The energy from the combustion of the wood radiates from the source of the reaction (fire is a chemical reaction, fyi) and you observe this radiation as heat on your skin.
This form of radiation (heat and light) is mostly harmless and can be observed in specific parts of the electromagnetic spectrum, more specifically, the infrared portion of the spectrum is particularly good at detecting radiating heat from objects. The electromagnetic spectrum is simply a chart of wavelengths that light (or energy) can be emitted or measured. Our eyes detect the visible portion of this spectrum and each color that we see is a result of reflecting (or radiating) energy that has specific wavelengths. Here is a good chart for understand the concept of wavelengths.
Below is a link to the source of the chart and more information about radiation, but I will explain some of the information from these links in subsequent posts.
More information about radiation can be found at these links, but I will be explaining the information in these links in subsequent blogs:
Nuclear Regulatory Commission: http://www.nrc.gov/about-nrc/radiation/health-effects/radiation-basics.html