Or, looking at it another way, it consumes 100 Watt Hours when it's been on for one hour. In usual units that would be 0.1KWH. The amount of power used in Watts is not related to time. When you are charged for electricity, the power you use it is multiplied by how long you use it for. Watts x Hours = Watt Hours.
100W is the power consumed. Power delivered for a period of time is a measure of the energy used. It is the product of power and time. In your case the energy used would be 0.1kWh
If you bring time into the equation then it is Energy not Power that you are measuring/calculating. So, a 100W bulb consumes 100W of Power. In one hour it will consume 100Wh (Watt Hours) or 360kJ (kilo Joules) of Energy.