From Bravo Fleet Infobase
Deflector Shields is one of many generic terms used for the protective energy barriers used to protect a ship during situations were the ship can be damaged or destroyed. These systems have also been referred to as deflector screens or just simply shields or deflectors.
There are several different shielding systems in use by Starfleet and they are as follows.
Standard Shielding System: The oldest and simplest system in use. Generates a simple graviton based shield around the ship.
Regenerative Shielding System: Similar to the Standard shields but equipped with extra graviton generators to prolong the shields operational time during hazardous situations.
Multi-Layered Shielding System: This system uses three nested layers of shielding to protect the ship during hazardous situations.