UV (ultra violet) is a portion of the electromagnetic spectrum just past violet which is at the short wavelength end of visible light as seen by the human eye. UV can not be seen by the human eye, but when it falls on certain materials it may cause them to become fluorescent (ie. glow) and reflect visible light back to our eyes.
Fluorescent lighting tubes generate UVA and the powder on the inside surface of the glass tube becomes fluorescent and glows to produce a very efficient form of lighting.
Fluorescent tubes in dance clubs have a special glass coating so only the safe part of UVA "black light" is transmitted into the room, making only some items become fluorescent and glow extra brightly.