What are red, blue, yellow etc? They are colours of visible light.
In just the same way, LW,MW, SW are colours of ultra violet light, but we can’t see them.
They are also known as UVA, UVB and UVC in that order.

The sun radiates all the colours of UV light as well as all the visible colours we can see.

LW (UVA) is often seen in clubs, dance halls etc. It makes your clothes, fingernails, teeth glow (fluoresce)
MW (UVB) has more energy – it sunburns you. It was used in tanning beds. It has medical applications.
SW (UVC) is more energetic still, but fortunately, the ozone layer blocks it from getting down to earth. It is widely used for sterilisation and germicidal purposes.

Years ago UV could only be produced by tubes. Now it can be produced with LEDs.

You need to protect eyes and skin when using SW for any length of time. You should never look for more than a second into any turned-on UV lamp or torch. (See UV Safety)

Some rocks (minerals) glow under UV. Most don’t. Some rocks fluoresce (glow) under SW, others under MW and others under LW. Some glow under more than one UV wavelength. How minerals glow depends on what impurities (called “activators”) are present.

Most rocks that glow under MW also glow under SW or LW, so many fluorescence collectors just have a LW and a SW lamp or torch. MW is for people who want to specialise.

Ordinary fluorescent lights work like this: Inside the tube SW UV is generated. The inside surface of the glass tube is covered with stuff (called phosphor) that SW UV causes to fluoresce bright white or whatever colour the fluorescent light is. That’s why they’re called fluorescent lights.