In a power surge, or a transient voltage, the voltage being supplied to a device will be higher than the device is built to support, and this can cause serious damage to sensitive equipment.
If the increase in voltage lasts for three or more nanoseconds (billionths of a second), it is called an electrical surge. If the increase lasts for only one or two nanoseconds, it is called an electrical spike.
Today, many people employ the use of surge protectors, which constantly check for an abnormal rise in voltage being supplied to a device. If the voltage rises higher than the acceptable level, the excess voltage is diverted to a ground wire which protects the appliance.