Ultraviolet radiation (UV) is a type of radiant energy, much like the light we see, but with a smaller wavelength and therefore higher energy. It is defined as the light in the spectrum of wavelengths between 40-400 nanometers. This means that it lies between visible light and x-rays on the electromagnetic spectrum as seen in Figure 1.
There are different sub-categories of UV radiation: Vacuum UV, Far UV, UVC, UVB and UVA. The first three mentioned are almost never encountered on Earth as they are absorbed by the atmosphere, however the last two, UVB and UVA are encountered very frequently.
UVA radiation is in the range of wavelengths 320-400 nm, therefore it is lower energy than other types of ultraviolet radiation, but can still cause harm to humans. Initial exposure to it gives a pigment-darkening effect to human skin (tanning), but overexposure can cause sunburn, toughening of skin, cataract formation, and other unwanted effects. This is the most commonly encountered UV radiation - accounting for 95% of the UV light hitting Earth - since it penetrates the atmosphere very easily. UVA light is used in tanning booths and phototherapy lamps.
UVB radiation is the more harmful of the two common UV categories, this is because it has enough energy to cause photochemical damage to cellular DNA. Both UVA and UVB are necessary for the synthesis of vitamin D, but it is important to be exposed to them in moderation. Overexposure to UVB light causes much of the same symptoms as UVA, but it is also known to be a cause of skin cancer. Most UVB light is blocked by the ozone layer, however due to the known holes in this layer there are concerns that this will lead to an increase in skin cancer.
UV is invisible to our eyes and the effects on our skin aren't immediate, so the consequences of overexposure can very easily sneak up on someone. However, there are many ways to protect against UV radiation:
There exists a "UV index" included in most weather forecasts which help people to prepare for the harmful rays. Environment Canada has a useful table for understanding the UV index, available here.