Deflector shields are a defensive measure employed primarily by star-ships, (although a number of other applications exist as well) to protect them from both the harsh environment of deep space, as well as attack by hostile forces.

History

The earliest deflector shields were simple magnetic fields projected around a ship to deflect incoming charged particles.  This partially protected astronauts from cosmic radiation, and provided some defense against plasma-based weapons.

The first popular enhancement was to add some ionized gas to the field.  This blocked more radiation, and allowed the shield to be used as a magnetic sail.

Later models added reflective nanoparticles that could defend against laser weapons.

Once flux pinning technology matured, deflector shields could resist kinetic attack as well. 

All these basic techniques of deflector shield defense were developed first during Era-0 in the Spozak, and during Era-1 in human space.  Later models continued to refine and elaborate on these principals.

In practice there are two main types of deflector shields:

Energy Shields

These are the most common type of deflector shield, designed to stop energy weapons (lasers, plasma-bolts, and the like), and keep out the hard radiation of space.  The main weakness of these shields is that they can admit solid projectiles, such as torpedoes, fighters, or asteroids. Energy shields are affordable, consume a moderate amount of power, and can be built small and light.

Barrier Shields

These are rare, expensive, require a bulky, generator, and use a lot of power to operate.  Even a shield of this type designed to contain only a single prisoner without any personal space can weigh a ton, and consume enough power to operate a star-fighter.  Naturally, they are used only to protect the most valuable targets.  The main advantage to their use is that barrier shields, unlike energy shields, do not permit the passage of solid objects.