ST Shields
Most starships in Star Trek use defensive shield technology to protect against enemy attacks. As the shields take damage, they become weakened and effects from enemy attacks bleed through with greater frequency.
Nature
Shields are usually depicted as a bubble surrounding a starship that is visible only when actively resisting an attack. During the Dominion War, shields seemed to conform to the hulls of the starships projecting them.
Most transporters cannot beam through combat-grade defensive shields.
Capability
The shields of starships are typically capable of withstanding several attacks from opposing ships of similar size and technology. Blocking incoming fire steadily depletes the protective capacity of a shield, the reduction usually being described as a percentage of its full capacity.
Shields often perform poorly against new threats. For instance, Federation shields performed poorly against the phased poleron beam weapons used by Dominion warships during early battles, but Federation engineers were able to modify their shield systems to provide protection in later encounters.
The size, shape, and operating frequency of a shield is typically adjustable. All of these factors can affect a shield's performance against various types of attack.
Star Trek shields handle radiant energy far more effectively than collisions with massive objects.
Even with fully operational shields, it is not unusual for ships to sustain some internal damage when hit by weapons fire.
Vulnerabilities
Shields operate at a specific frequency. This allows friendly weapons fire originating from the shielding ship to pass through. If an attacker can detect the ship's frequency, such as in Generations, shields are ineffective. Many species such as the Borg, Vidiians or Voth, either explicitly or implicitly bypass shields in this manner.
The effectiveness of countermeasures depends greatly on the competence of the tactical officer and bridge crew. In Voyager "Equinox," Tuvok altered the shield frequences without prompting from Janeway. However, in Generations, Riker did not order shield frequency rotation despite previous experience with the Borg.
In The Next Generation "The Wounded," O'Brien exploited a vulnerability in U.S.S. Phoenix's shields to transport himself through its shields.
The Emergency Medical Hologram was able to monitor the shield frequency of U.S.S. Voyager from sickbay in Voyager "Equinox."