Conventional light bulbs are filled with an inert gas like argon. In a normal filament bulb, you are limited to a certain temperature because the tungsten evaporates from the filament, condenses on the cool glass envelope and eventually the filament breaks down. A very bright bulb would simply wear out too quickly. A halogen bulb enables the filament to run at a higher temperature, hence brighter.
Halogens are among the most reactive elements known.
The halogens are Fluorine, Chlorine, Bromine, Iodine and Astatine. When the tungsten evaporates from the hot filament it forms a vapour which cools as it nears the outer glass. As it cools it reacts with the iodine to form tungsten iodide. The tungsten iodide itself is stable until it comes into contact with the very hot filament, where it dissociates into its elements. Tungsten is redeposited on the filament and iodine vapour is released to repeat the cycle.