Surge is an increase in voltage in an electrical current that causes damage to electrical equipment. If an electrical current above the rating were to come through a power outlet for more than three nanoseconds this would be considered a surge. Anything less than three nanoseconds is considered a spike.
Spike is an increase in the amplitude of a signal for less than three nanoseconds. Anything more is considered a surge. A power spike can damage any electrical component.
Surge protector is a plug-in device, designed to protect any electrical equipment from surges and spikes. It is also known as a surge suppressor.
Voltage drop is decrease in voltage in less than one second. It is also known as brownout, dip, voltage reduction, and voltage sag.
Under voltage is reduction in voltage for more than one second.
Voltage drop or under voltage damage your electrical equipment like computers. To prevent damage, use UPS.
UPS means Uninterruptible Power Supply. It is a device, to provide backup power source in case of a power failure (outage /blackout), brownout, or a surge in power.