Ultraviolet lights and black lights are essentially the same thing. But the terms can be used to discuss different forms of ultraviolet light. The three forms of ultraviolet light are UV-A light, UV-B light and UV-C light.
Black light and ultraviolet (UV) light are synonyms that refer to the same phenomenon. In colloquial English, however, the term black light is often applied to party or decorative lights, while UV light refers to lights used for a different purpose.
Black or UV light is comprised of light at the end of the visible spectrum and beyond. Blue and purple light (light with the shortest wavelength) can sometimes be seen in a black light. The rest of the light (UV radiation) is invisible because its wavelength is so short.
UV-A light is often referred to colloquially as black light. This is the most visible type of UV light, because it has the longest wavelength. You can buy UV-A black light bulbs at most stores. They release bluish-purple light and are used in concerts and entertainment because certain colors appear to glow beneath them.
UV-B and UV-C lights are more often referred to as UV lights than black lights. UV-B lights are medium-wavelength lights that release less visible light than UV-A lights do. UV-B lights are used in forensics, because certain human fluids are revealed under the lights. Police can use these lights, for instance, to find blood or fluids at a crime scene.
UV-C lights have the shortest wavelength of all UV lights and are effectively invisible to humans. UV-C lights emit destructive radiation that kills germs. UV-C lights, therefore, are mostly used for sterilization and water purification purposes.