The deflector shields, often referred to as just shields, or less often referred to as screens, are force-fields that surround a starship, space station, planet or other facility for protection against natural hazards and enemy attacks. Weapons fire, either energy or projectile-based, cannot directly penetrate the shields, unless the shield frequency is known and exactly matched. Without matching, weapons may weaken the shields by draining energy from them and their respective energy sources. Transporters cannot be used when the shields are in place.
There are two common shield configurations: Ellipsoid and Contour-conformal. Ellipsoid configuration projects an ellipsoid shield bubble around the ship and regions directly adjacent to the ship. This shield is presumably stronger than contour-conforming, but creates a larger target profile. Contour-conforming creates a shield layer mere meters away from the hull, allowing for a slim target profile. Because of the complexity of this configuration's geometry, it is generally weaker than ellipsoid configuration.
Amongst the different types of shields are the standard type which allow both phaser and photon torpedoes to be fired but will not allow any transporter activity. Another type of shield will allow photon torpedoes, and Transporters to be used, but will not allow phasers to be fired. One type of shield was capable of sustaining the energy of ninety Photon torpedoes, although it would appear that this type of shield was a failure because it was not used often.
It is possible for a starship to have only certain areas of the ship shielded, leaving other areas unprotected. This would be useful in times when power reserves were low, and only certain areas could be shielded. This was also useful to Captain Janeway on the USS Voyager, when she wished to extract information from a renegade crewman concerning the whereabouts of Captain Ransom, in 2375. In this instance, she lowered the shields around a Cargo Bay, with the crewman inside, to allow the nucleogenic creatures to attack him. Additionally, a vessel can extend their shields around another vessel by matching shield emitter frequencies. Lowering portions of the shields also allows usage of transporters without completely sacrificing the protection that shields provide.
The type of deflector shield used by the Lysian Central Command had an output of 4.3 kilojoules and could have easily been taken out with a single photon torpedo.
Shield article at Memory Alpha.