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A surge protector is an appliance designed to protect electrical devices from power surges and voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or shorting to ground voltage above a safe threshold.

Surge protectors can be built with one or more of the following electronic components:

A fuse burns out or a circuit breaker trips when excessive power is being consumed and fed to a device. These can protect both from surges caused by the device itself and from external surges that last long enough.
An iron poor transformer can transmit AC power similar to a normal iron core transformer (although less efficiently), but will be unable to transmit sudden surges that saturate the small iron core.
A MOV is a small device that will short out when presented with a sufficiently high voltage, hopefully passing the surge to ground through the MOV rather than through the protected device. Unfortunately, these devices tend to self destruct (and are therefore one-shot devices) with sufficiently strong surges. Many cheap surge protectors only have MOV's, and probably will not survive even a single lightning strike.
A zener diode is a small diode designed to protect against normal spikes in a circuit, especially motor controller circuits. These are sometimes paired as a transient voltage suppression diode.
A gas discharge tube is used much like a MOV, except that it relies on a trapped gas to become ionized to pass the voltage. This has the advantage of being able to pass much more power without self destructing, but the disadvantage of reacting to the high voltage more slowly.
An Uninterruptible power supply (in addition to other surge protection devices listed above) typically passes external power past the unit's battery, which will absorb spikes much like a capacitor in parallel acts as a low pass filter. (This also keeps the battery charged, and in the event of a failure, the battery can continue to supply power without any interruption otherwise needed to switch it in.) Typically these are the best ones, but are also more expensive.

External link

HowStuffWorks: How Surge Protectors Work

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 Surge protectors can be built with one or more of the following [[electronic component]]s:
 * A [[Fuse (electrical)|fuse]] burns out or a [[circuit breaker]] trips when excessive power is being consumed and fed to a device.  These can protect both from surges caused by the device itself and from external surges that last long enough.
-* An iron poor [[transformer]] can transmit [[alternating current|AC]] power similar to a normal transformer (although less efficiently), but will be unable to transmit sudden surges that [[Saturation|saturate]] the small iron core.
+* An iron poor [[transformer]] can transmit [[alternating current|AC]] power similar to a normal iron core transformer (although less efficiently), but will be unable to transmit sudden surges that [[Saturation|saturate]] the small iron core.
 *A [[Varistor|MOV]] is a small device that will short out when presented with a sufficiently high voltage, hopefully passing the surge to ground through the MOV rather than through the protected device.  Unfortunately, these devices tend to self destruct (and are therefore one-shot devices) with sufficiently strong surges.  Many cheap surge protectors only have MOV's, and probably will not survive even a single [[lightning]] strike.
 *A [[zener diode]] is a small diode designed to protect against normal spikes in a circuit, especially motor [[controller]] circuits.  These are sometimes paired as a [[transient voltage suppression diode]].