UV is short for ultraviolet. It refers to energy (technically 'electromagnetic radiation') that sits somewhere between visible light and x-rays.
When we talk about UV rays we are generally referring to energy that is coming from the sun. As we all now know, UV is the cause of sunburn, which can lead to early aging of our skin or even skin cancer if over exposed.
UV rays have a wavelength which is shorter than visible light, which is why you cannot see them (just like x-rays which have an even shorter wavelength). They can be detected however using specialized sensor equipment designed to see those shorter wavelengths.
Most of the harmful UV rays are filtered out by our earth's atmosphere. So as a result, we remain protected from the harmful effects of UV. It's a very different case for astronauts however who require some serious sun protection in space.
Due to the drastic increase in pollution though, holes have been created in our atmosphere. These holes in the 'ozone layer' have exposed us to more of these harmful rays, particularly in certain parts of the world. The long term effects of which are resulting in a greater occurrence of skin cancers.
It's not all bad though, as some sunlight on your skin is beneficial. Not only does it affect and improve our mood (psychological wellbeing) but our bodies also need that sunlight directly on our skin to produce vitamin D.
You just have to be sensible as only a small amount of direct sunlight is necessary, so you can still avoid the sun and wear subscreens most of the time.
UV light is not just about sun tans, sun burn and skin cancer though. There are also some artificially made sources of UV which we use to our advantage.
'Black lights' and 'Ultraviolet LEDs' can be used for killing bacteria and sterilizing medical equipment or surfaces. Many bacteria and viruses can't survive exposure to ultraviolet light. You can actually use this trick yourself. Have you ever thrown away a washing up sponge because it started to smell? That's bacteria building up in the sponge. Instead of throwing it away, try leaving it outside in the sun for the day - and let the natural UV light fix the problem for you.
Ultraviolet rays can also prove to be useful in the treatment of certain conditions, such as rickets which is caused by a lack of vitamin D. Or vitiligo which is a discoloration of the skin due to a lack of melanin.
UV rays also bear a special ability to speed up many chemical reactions (as a catalyst), and can be used to cause fluorescence in some materials. You may have seen CSI operatives on TV using a black light to make invisible biological substances 'glow'. This does actually work with certain fluids such as saliva or urine, or even tiny fragments of bone or tooth. It doesn't work however with blood - that's a bit of TV trickery. It is possible to make blood show up in the way you see on TV, but it's not done with a black light.
'EEPROM' memory in computers can also be erased on exposure to UV rays. This results in the reusability of this special kind of memory chip, which allows them to be store information without power but also still be overwritten with updates.