Ultraviolet light, or UV light, is a type of electromagnetic radiation that has a wavelength somewhere between visible light and X-rays. It is widely used throughout the world, in everything from production of usable electricity (the sun's rays are ultraviolet) to the many common uses for a simple black light.
All sun tanning beds use ultraviolet lights in order to simulate the effects of sunlight against bare skin. Just like real sun bathing, people who use tanning beds run the risk of developing sunburn.
Many hunters will use an ultraviolet light, such as a black light, to locate and follow a wounded animal. This is done by shining the light on the ground and following the illuminated blood trail.
Forensic scientists will also use ultraviolet lights to look for clues at a crime scene.
CD and DVD players use lasers, which are a form of ultraviolet light, to read information off of the disc.
To fight counterfeiting, many government or otherwise sensitive documents will have some sort of ultraviolet verification. In other words, if you shined a black light over a $20 bill, you would see markings on an authentic bill that you wouldn't find on a fake.