What is the function of the glass bulb in a bulb?

On a standard incandescent light bulb, the glass traps a set of gases around the filament that help keep the filament from burning up quickly.

(The filament is the part that does the actual glowing inside the glass.)

It also keeps the oxygen in the air that we breathe away from the filament. Oxygen is one of the gases that will vastly accelerate the destruction of the filament.

(Also, the glass keeps anything from touching the What_is_the_purpose_of_the_glass_on_a_light_bulbconductors inside.)

The glass can also act as a filter to remove any ultraviolet radiation caused by certain types of light bulb's ("lamp's" technically) methods of creating a arc to produce light.

When specially treated, the glass can also be used to break up the light rays to cause more diffused source of light. (Think "Soft White" to get an idea.)