What is the difference between a 100watt and a 75 watt bulb?

The amount of light produced and the amount of power consumed. A watt is a unit of power and electrical power is determined using the formula: P = I2*R , Where P is power, I is amps and R is resistance. But using Ohm's law ( V = I * R) the following can be obtained: P = V2/R , Where V is voltage. In US house holds the standard voltage is ~120V, changing the resistance of the filament of the light bulb will change the amount of power consumed and therefore the amount of light produced. The higher the wattage the lower the resistance of the filament. Filaments are used in standard incandescent light bulbs and not in fluorescent tubes or LED fixtures.