UV-light is very, very violet. Actually it is so violet that you can't see it, but you can get a sunburn from it. So on its own UV-light wouldn't make a useful lamp, that's why there is a phosphorous substance in the inside of the glass tube. When UV-light hits the phosphor atoms, they absorb the UV light and send out the white light that illuminates your room. The conversion of light from one type to another is called fluorescence, which gave the fluorescent lamp its name.
Fluorescent lights conserve energy.
For the same amount of light they need less power than usual light bulbs. By the way, the funny shaped light bulbs made of bend glass tubes in the supermarket are actually fluorescent lamps. Please don't play and break fluorescent lamps they contain chemicals that are poisonous.