Fluorescent lamps are lamps which use the effect of electricity on Mercury vapour within an enclosed glass tube or bulb to produce light.
The tube contains two electrodes, one at each end, and is filled with an inert gas - usually Argon. The inside of the tube is coated with a phosphorescent substance, and it also contains a small blob of Mercury.
When a fluorescent lamp is switched on, as long as there is a sufficient potential difference, an electrical current will flow between the electrodes. As the electrons which make up the current move, they have sufficient energy to vaporise some of the Mercury.
Collisions between the electrons and the now-gaseous Mercury atoms cause electrons within the Mercury to "jump" momentarily to a different place in the atomic structure, but they cannot maintain this position and so "fall" back down, emitting the extra energy as photons of UV light, which is absorbed by the phosphorescent material on the walls of the tube/bulb, and re-emitted as visible light.
Fluorescent lamps thus operate differently from filament lamps, halogen lamps, or LEDs: their main advantage is that they are much cooler in operation, because the electrical energy supplied is mostly converted into visible light, not heat.