Understanding electrical consumption
The issue you're confused about seems to be the difference between power and energy.
Energy is how much work you can do. Common units are joules or watt-hours.
Power is how fast you do work. It's a rate of change. Common units are watts or horsepower. Horsepower is probably an instructive unit to consider. Say you wanted to move a large pile of straw. Whether it's moved by a horse or a housecat doesn't affect the amount of work done. But the horse does it faster, because it's a more powerful animal.
For the purposes of discussing grid electricity consumption, watts (W) and kilowatt-hours (kWh) are the most common units used. To know how much energy is consumed, multiply the power by the time. 100 W x 1 hour is 100 watt-hours, or .1 kWh. In short, the relationship between power and consumption is time.
A 100W bulb consumes 100W assuming the voltage across it is what's specified on the package, which is usually 120V in my experience. If the voltage at your socket is lower, the bulb will consume less power. It's approximately a fixed resistance, so the power consumed is $$ P=\frac{V^2}{R} $$
As an aside, remember energy conservation. If something consumes 100W, that energy is being converted to some other form. Either it gets stored (potential energy), it's used (light, motion, etc.), or it's wasted as heat. For an incandescent bulb, ~90% of the power consumed is converted to heat. So a 100W incandescent bulb consumes 100W, but only outputs 10W of light. It gets hot because the other 90W is being wasted. Which is why CFL's run so much cooler and consume less power for the same light output.
The others already told you that energy is power \$\times\$ time. And that 1 kW during 1 hour is 1 kWh. A year has 8765 hours, then a device which is in standby most of the time will consume 8.76 kWh in a year if it consumes 1 W in standby.
The kWh is a practical unit for electrical power, but the SI unit is the joule ("J"), which is much smaller. A joule is the energy consumed by a 1 W device in 1 second.
\$ 1 J = 1 W \cdot s \$
So a kWh is 1000 W \$\times\$ 3600 s = 3.6 MJ (megajoules), so that's not so practical, especially when we talk about industrial machines consuming tens of kWh; you would soon be talking about GJs (gigajoules). That's why the kWh is accepted as alternative unit.
(An older unit for energy is the calorie, where 1 cal \$\approx\$ 4.2 J. An odd number, but it's the energy needed to heat up 1 g of water by 1 °C (from 14.5 °C to 15.5 °C). You can do that by adding 4.2 W during 1 second, or 42 W during 100 ms.)
The power rating on the light bulb is the rate at which it consumes energy. How much energy is consumed is a function of how long it is turned on. Energy = Power × time. If it is on for one hour, it will consume 100 Watt Hours. This is the unit that the electric company bills.