Passive shield
While all characters have access to a bubble shield, Link and his counterparts carry their own personal shields while in battle. While standing still, incoming projectiles that hit the shield will deal no damage and simply push Link back slightly. This effect was implemented in Melee and carries over into Brawl.
Since projectiles must hit the front of the shield to be blocked, Link may still be hit in the back, head, or feet, and parts of his standing animation may leave the shield temporarily useless. Crouching can migitate this somewhat, but then any projectiles that would have hit the shield normally will instead hit Link in the face. Link's shield is also unusable when he is holding an item.
The design of each Link's shield is taken from the respective game. Link carries the Hylian Shield from Ocarina of Time in both Super Smash Bros. and Super Smash Bros. Melee while he uses the shield's Twilight Princess design in Super Smash Bros. Brawl. Young Link uses the Deku Shield from Ocarina of Time (which does not burn like its original counterpart), while Toon Link holds the Hero's Shield from The Wind Waker.
List of items that can be blocked in Brawl: Capsule, Rolling Crate, Crate, Party Ball, Ray Gun(shots and thrown), Super Scope(shots and thrown), Beam Sword(when thrown), Banana Peels, Blast Box(when thrown), Bom-omb, Cracker Launcher(shots and when thrown), Fan(when thrown), Fire Flower(flames and when thrown), Freezie, Green Shell, Hammer heads, Home-Run Bat(when thrown), Hothead, Lip's Stick(when thrown), Motion-Sensor Bomb(when thrown), Mr. Saturn, Poke Ball, Smart Bomb(when dud), Smoke Ball, Soccer Ball, Star Rod(stars and when thrown), Superspicy Curry, Unira(when thrown)
List of specials that can be blocked in Brawl: Aura Sphere, Banana Peel(Diddy Kong), Blaster(Wolf, Falco, and Fox), Blizzard, Bomb(Link and Toon Link), Boomerang, Bullet Seed, Chef, Din's Fire, Egg Throw, Final Cutter(shock wave), Fire Breath, Fireball(Mario and Luigi), Flamethrower, Force Palm(projectile), Gale Boomerang, Gyro, Hand Grenade, Hero's Bow(Link and Toon Link), Ice Shot, Missile, Needle Storm, PK Fire, PK Flash, PK Freeze, PK Thunder, Palutena's Arrow, Paralyzer, Peanut Popgun, Pikmin Throw(only purple Pikmin), Razor Leaf, Robo Beam, Spring Jump(spring when used in air), Super Dedede Jump(stars), Thunder(lightning), Thunder Jolt, Toad(spores), Vegetable, Waddle Dee Toss(only Gordos), Wario Bike(bike when thrown and tires), Yoshi Bomb(stars)