Plasma shield

Plasma shields protect Protoss warriors, combat vehicles and most military structures.

History
During the Aeon of Strife, Protoss warriors used focused psionic energy to surround themselves in impregnable energy shields. Over time, Conclave scholars and Templar sages learned to reproduce the energy shield using induced psi-field generators, which allowed even the smallest robotic machine to surround itself with a protective field.

Gameplay
Shield points act as another form of hit points and must be lost first. Unlike regular HP, plasma shields always suffer full damage regardless of the type of attack used against them. For instance, if a Ghost, whose weapon deals concussive damage attacks a Dragoon, a Large unit, its shield points will suffer the full damage, whereas the Dragoon's real hit points will only suffer one quarter the damage.

Shield points regenerate at a fairly slow rate. Shield Batteries can be accessed to increase the rate of this regeneration.

Shield points always have base "armor" of 0 (which is separate from the armor protecting the regular hit points). This "armor" can be upgraded at the Forge (up to three times). Because Protoss units nearly always have both shield points and hit points, it is important to upgrade both types of armor. This also applies to buildings, making Protoss the only race able to increase the defense rate of their buildings.

Weaknesses
The Terran Science Vessel has the EMP (Electromagnetic Pulse) ability, which drains all kinds of magnetic belt-created energy. In-game it drains all energy points for all three races as well as all Protoss shield points.

Shields in StarCraft II
In StarCraft II, shield points regenerate out of combat more than twice as quickly as in StarCraft I.

Karune indirectly suggested that bonus damage is applied to shields (based on the properties of the unit protected by the shields). Zealots have a combined hit point and shield point total of approximately 160. Ghosts deal 5 base damage (+30 vs lightly armored units such as Zealots). If Ghosts did not deal the bonus damage against shields, it would take the Ghost at least 11 shots to remove the shields, but Ghosts only need 5 shots to kill the Zealot (removing all shield points and hit points).