Shields generally allow a given ship to absorb projectiles and destroy weak enemies on contact a limited number of times before disappearing.
Shield: A pair of gear-like objects fly in from the right side of the screen to attempt to attach to your ship, providing frontal protection. Absorbs 15 hits, and usually shrinks with each hit.
Forcefield: Your ship is enveloped in an aura that absorbs hits from all sides. Usually absorbs 3 hits before disappearing, but some titles grant more hits.
Multi-Shield: Each shield powerup bestowed upon the ship places a single gear at one of the 4 cardinal directions.
Free Shield - Same as Multi-Shield, except the gears fly in from the right, and can be attached to any part of the ship.
Rotating Shield - Two gears at opposite points rotate around the ship rapidly.
Reduce - In Gradius III arcade, shrinks the ship (and the collision box), and can be upgraded once for a smaller size. In Gradius III SNES, it shrinks the ship and absorbs up to two hits like the Forcefield, with each hit enlarging the ship towards its normal size. PS2 and PSP versions of Gradius III arcade provide a Reduce II shield that mimicks this function, and a version of Reduce I that fits in the exclamation cell.
Guard - A Gaiden exclusive that protects top and bottom only from two hits. With this shield active, terrain collision is impossible.
Limit - A shield that simply grants a few seconds of invincibility. This allows the ship to pass through terrain.
Megacrush - Generally an exclamation cell exclusive, but Gradius V has it as an option for the shield cell. Activation clears the screen of weak enemies and all projectiles.