A power surge is a sudden brief increase in the line voltage, usually caused by lightning or power outages, short circuits, tripped circuit breakers, etc. It can damage you electrical equipment or at least reduce its life span. When lightning strikes a power line, it creates an extremely large power surge that overpowers almost any surge protector, so don't rely only on a surge protector. The best way to protect your computer is to unplug it. Also unplug any phone or coaxial cables.
To tackle voltage fluctuations, it's advisable to use a good quality surge protector. The quality is generally proportional to the price, a cheap spike guard would do little more than just providing power outlets. It should also have an indicator light indicating whether it's functioning properly or not. It should have a fuse that burns out if more than a particular amount of voltage passes through it. Power surges can also travel through your phone lines and coaxial cables-the phone lines connects to the modem and the coaxial cable is used to provide broadband Internet. Look for a surge protector with a telephone and coaxial cable line-in-jack. Some UPS units also provide surge protection.