If it's an incandescent, then it's nothing more than a filament suspended with wire between two conductors in a bulb filled with a gas mixture.
If it's a fluorescent, there will be a small circuit which just rectifies the mains to DC, then runs it through an oscillator and transformer to make about 400VAC. The transformer typically has a higher voltage winding which is used to start the bulb at a higher voltage. Here's a great page which describes the typical circuit: http://www.pavouk.org/hw/lamp/en_index.html
If you are referring to LED, then typically the mains is rectified to DC and then sent through a PWM circuit to regulate the voltage for the LEDs.