It's also good to know the difference between Power and Energy.
Power is the rate at which energy is being produced, energy is the total amount of energy produced over time. I use an automobile speedometer to explain the difference.
Power is the "needle on your speedometer telling you how fast you're driving at this moment. Energy is like the odometer telling you how many miles you have traveled.
A generator producing power at 100MW at a constant rate will produce 100MWh of energy over one hour.
The net amount of energy received at the end of the transmission line, after step down is less than the 100MWh due to system losses.