Voltage spikes occur occasionally, through lightning strikes, power outages, short circuits, or electromagnetic pulses, causing short, but fast bursts of energy and these can ruin electrical devices. If not curbed or grounded and nullified, these voltage spikes can become voltage surges. Surge protectors are appliances that protect electrical devices from such spikes, by blocking or shorting voltages that are above a safe threshold. Many power strips, which are blocks of electrical sockets that allow power to multiple devices, have built-in surge protection; power blocks should not be confused with surge protectors, as they are so often mistakenly called.
The most common type of surge protector features a piece called metal oxide varistor, or MOV, is comprised of three components: semi-conductors made of metal oxide stationed between the power and grounding lines. Low voltage causes the electrons in the MOV to have a high resistance; conversely, when the voltage is high, these same electrons create a much lower resistance, allowing the extra voltage to pass into the ground until the levels are normalized. In addition to the MOV, some protectors have built-in fuses, which resist and conduct current as long as that current is below a certain threshold. Once that threshold is breached, the resulting heat burst the fuse, cutting the circuit. Fuses are a one-time use unit and should be replaced as soon as possible after being destroyed.
For more information on how surge protectors work and how to choose the best for your use, check out this article