what is a fluorescent bulb?

The fluorescent bulb is used to the convert the electric energy into the light energy efficiently than other bulbs. It is gas-discharge bulb.

The electrical energy from the power supply generates electrons from the bulb electrodes.  Emitted electrons collide with mercury atoms present in this bulb giving invisible UV light.

Finally, UV light is converted into visible light by white phosphor coating of the bulb that we can see.