Shields, referred to as barriers in Japan, are a series of power-ups that protect the player from harm. When the player runs into an enemy or trap, their character will be knocked backwards and the shield is lost. Shields offer an extra line of defense before rings would be lost. Shields can be obtained by breaking monitors, or in the gumball machine (Sonic 3) and the sphere shot (Sonic & Knuckles) bonus stages.
The original shield is a simple blue sphere, with no additional advantages. The one exception to this is in Sonic 3D: Flickies' Island, where the basic shield is impervious to the electric flooring in Gene Gadget Zone.
In Sonic 3, the basic shield was completely replaced by a trio of "elemental" shields: lightning, fire, or water. All three still protect against a single hit, but these shields are also able to protect against most projectile attacks, reflecting them with no damage to the shield. Each shield has a unique passive effect, and Sonic (only) is able to perform a special attack unique to each shield by pressing Jump while already in the air. If the player collects a shield while under the effects of a super transformation, the shield graphic will not appear although its secondary effects will still apply (i.e. ring magnetism with the lightning shield).
Sonic 3 also features something called the Insta-Shield, also known as the Twin-Spin Attack. This is not another shield, but a fourth special attack for Sonic that's performed by pressing Jump in the air without a shield. It is an improved version of the normal Spin Attack, increasing Sonic's attack radius and even making him invulnerable for a split-second - very useful for dispatching Orbinauts.