Pyrotechnics

Pyrotechnic devices, or explosive ordnance, are used on spacecraft for many applications.  They have high energy density and are compact, easily stored, and controllable.  However they are only used reluctantly.  When another option is available generally it will be selected over pyros.
Pyros are single-use devices, once set-off there is no retrieving them and the system cannot be reset.  Pyros can only be used for a component that begins in one configuration and then changes to another and remains that way for the rest of the flight.
They produce very short, impulsive loads.  Despite their high energy density all that energy is expended at once.  Pyros don't release their energy in a controlled fashion and controlling the energy release involves additional structure and engineering.
A pyrotechnic device cannot be functionally tested, there is no way to be certain it works until it is used.  There are very few tests that can be done to ensure a charge functions before installing it.  Reliability of pyros is based on qualification.  If we get a box of 100 then we set of 20 of them.  If all of them go off we assume there's a good chance that the next one will when we use it.
If something does go wrong with a pyro during flight there's very little that can be done about it.  Failures can and do happen, and when they do the vehicle is already off the ground.  Redundancy can be used to mitigate failures, so that if one pyro doesn't go off another can accomplish the same job.
Pyro usage falls into three broad categories: operational, flight termination, and emergency use.  Operational devices are used as a standard part of the flight plan.  Emergency use devices are activated during contingencies, to regain control of the vehicle or attempt to salvage some critical system (like the crew).  Flight termination devices are used to safely abort failed flights.  An aborted flight is destroyed and/or sent into the ocean, to reduce the damage it does as much as possible.
Pyros are classified into two categories: category A, hazardous, and category B, nonhazardous.  Category B devices could go off in your hand without causing any injury.
With any explosive device we have two chief concerns: inadvertent firing and nonfiring.  Either of these could result in mission failure, since each step of the mission is time dependent.  Both are also serious range safety concerns.  An early firing could happen while technicians are on the range working.  A nonfire means a primed explosive is still sitting on the range, and no one knows when it might finally go off.
To minimize risk a number of requirements are in place for pyros.  They are always installed as late as possible during the assembly process.  After installation inhibitors are put in place to prevent the devices from going off, and are not removed until just before launch.
Once the pyros are in place radio broadcast at the range is limited so that the radio power at any device is less than 20dB under the no-fire power.  Electromagnetic waves can induce a current in a wire, so radio power must be kept low enough that the induced power is always below the lowest level that the devices will fire at.
During installation there is full radio silence on the range.  No other operations take place during pyro installation and all range personnel wear flame retardant, nonstatic coveralls.  All equipment and personnel must be grounded.  Pyros have faraday caps, metal covers that provide a voltage path around the charge.  All lines are checked for stray voltage prior to installation.  There must be a clear area of at least 10ft around any work with pyros.  Humidity must be at least 35% and there cannot be a storm within 5mi.  All these precautions are taken to ensure no stray spark accidentally triggers any explosive device during installation.
Pyrotechnic systems are designed to be completely separate from all other systems.  A pyro system has shielded circuits with a single common ground.  All pyros are built with metal connectors that cannot mismate.  The pyro system consists of the following, separate and independent, systems:

• power source - a dedicated battery or capacitor independent of the main power bus.
• firing circuit - circuit that connects the initiating device, or detonator, to the power source.  FIRE and ARM circuits are required to be separate, so that if either is accidentally triggered there is no ignition.
• control circuit - activates and deactivates the safety devices
• monitor circuit - monitors the firing circuit for stray voltage.  This must be completely independent of the control and firing circuits.

 The SAFE and ARM plugs are also required to be as close to the device as possible.  This reduces the number of locations for possible failures.

That gives you a rough idea of just how serious the safety concerns are when handling and installing pyrotechnic devices.  Now we'll look at a few of the ways they are used.
Initiators are used to trigger another device.  They can be used to ignite a solid rocket motor or to set off another pyrotechnic device.  The NASA Standard Initiator (NSI-1) is seen below, connected to the NSI-Detonator.  The detonator amplifies the explosive force of the standard initiator.

Frangible nuts are a category B ordnance used to uncouple structures.  They are ordinary nuts with breaking points added.  These breaking points are stuffed with explosives, so that when the pyro is triggered the nut breaks along the existing failure line.

Separation bolts serve a similar function.  They are hollow, with explosive charge built in and a designed failure point.  When the explosive goes off it pushes a pin, which breaks the bolt at the defined point.

Pyrotechnic linear actuators push a rod out.  The rod is contained in a hollow chamber with a small explosive charge.  When the charge goes off the pin is pushed out by the force.  Spring-loaded pins keep the rod in place once extended.

Pin pullers do the opposite job, they retract a rod.  In this case the rod ends in a plunger, with the explosive charge between it and the wall.  The explosive pushed the plunger back and the pin withdraws.

Pyrotechnic valves switch from open to close, or closed to open, once.  Normally open valves switch to closed, by pushing a plug into the flow.  Normally closed valves have a plug with an opening above in the flow, it is pushed so that the opening allows fluid through once activated.  Both of these pictures are normally closed valves

Shaped linear charges hold an explosive in a metal container, which expands when the explosive goes off.  Mild detonating chord contains a linear core of explosive material contained in a metal sheath.  Confined detonating fuse uses a combination of fiberglass outer sheath, rayon middle sheath, and lead inner sheath with layers of vinyl between.  CDF contains an explosion once the linear charge goes off.

Super*Zip is another method of severance.  Two sheet metal faces surround a core structure.  At points the core is replaced with CDF, as the CDF goes off its outer sheath expands.  This pushes the two plates out until they break, separating the structure.  It gets its name because it resembles a zipper being unzipped.