In a light bulb the filament is a thin wire that glows

when a current flows throgh it. The first light bulbs used a carbon thread as a filament. Today tungsten

filaments is the most used due to its high melting point.

Most light bulbs contain some inert gas since

the tungsten filament vaporizes quickly in vaccum.

the most used gas is argon and nitrogen. But in some lamps
the argon is replaced with another gas
, like in the halogen lamps and krypton bulbs.