A watt-hour is a unit of electrical energy, often used in determining cost for electrical usage. It is the result of one watt being consumed for the duration of an hour.
The formula for watt-hours is simple. If a 10-watt device is running for an hour, then that device has consumed 10 watt hours. If it is a 100-watt device, and it has been running for a half-hour, then it's 50 watt-hours of consumption.
Watt-hours are measured by electrical meters.