There are several types of protective devices used in power systems:

Overcurrent protection: fuses and circuit breakers.

Overvoltage protection: usually spark gaps.

Electric shock protection: usually ground fault interrupters

Arc fault protection: called arc fault circuit interrupters.

Overcurrent protection is intended only to prevent fire and
equipment damage. It takes the form of a fuse or circuit 
breaker on the hot side between source and load.

Overvoltage protection is seen on the hot side of transmission lines.
It's intended to protect against lightning strikes and maybe other,
higher-voltage conductors coming into contact with lower voltage wires.
It usually takes the form of a device that shorts the hot side to
ground on overvoltage, thus tripping an upstream overcurrent protection 
device. Typically it'll be a spark gap or metal-oxide-varistor.

It's worth noting that overvoltage protection devices are usually
one-time-only.  Disconnecting a load under overvoltage requires a 
high-voltage opening switch that is fast acting. The only reliable 
device for the job is a fuse. So-called "surge" protectors are 
usually metal-oxide varistors without any upstream fuse and so are 
generally ineffective. Even with a fuse, the overvoltage can't be 
halted until enough energy is absorbed to melt the fuse. That's too 
late to protect solid state devices which are directly connected. 

Electric shock protection is normally accomplished by carefully
measuring current balance between hot and neutral. Any imbalance
is interpreted as a fault and triggers a disconnect. The advantage
is that imbalances smaller and briefer than human injury threshold
can be detected, allowing the most common electrocution scenarios
to be avoided. Though called a ground fault circuit interrupter
it has nothing to do with grounds and grounding.  

Arc fault protection is relatively new. It relies on the fact that
insulation failures (and sometimes loose or faulty connections) can
generate high frequency electrical noise. When the high frequency
noise exceeds a threshold a disconnect is tripped. Switch contacts
can generate very similar electrical noise which makes deployment
of arc fault protection prone to nuisance tripping.  

It's somewhat noteworthy that protective devices are not closely
related to grounding. Ideally, circuits can be safe without reference
to ground. But, that requires perfect insulators. Sice no insulator
is perfect, insulation faults must be taken into account and that
leads to the need for grounding and bonding. 

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