You can also view previously answered questions in the HSW Question Archive . These bulbs have a very thin tungsten filament that is housed inside a glass sphere. The basic idea behind these bulbs is simple.
Electricity runs through the filament.
Because the filament is so thin it offers a good bit of resistance to the electricity, and this resistance turns electrical energy into heat. The filament literally incandesces because of the heat. The problem with incandescent light bulbs is that the heat wastes a lot of electricity. Heat is not light, and the purpose of the lightbulb is light, so all of the energy spent creating heat is a waste. Incandescent bulbs are therefore very inefficient.