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 and Smash 4, with the exception of Smash 4 having no knockback, and an animation when something is blocked. Another change in Smash 4 is Link being able to use his shield while walking, but not dashing. Toon Link can also block while walking, but only at a very slow speed.
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 in Brawl may leave the shield temporarily useless. Link's standing animations were changed in Smash 4 so he will always block projectiles while standing. Crouching can help cover the gaps 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 or if he's in the air.
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 and Super Smash Bros 4. 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.
To consider the projectile to be purposed for the shield, it must be reflectable and not unblockable.
Link has always possessed a shield in every Legend of Zelda game to defend against enemies and solve puzzles. Some games had the shield active whether he was standing or walking. Or it might had to have been used as an item. Link needed to be facing the attack to block it. Some attacks are too powerful for the shield making link stagger or take damage. Some enemies can even steal his shield, making him have to find or buy another one.
The design of each shield is taking from their respective games. Ocarina of time shield was in Smash 64 and Melee, and the Twilight Princess shield in Brawl. Young link holds a Deku Shield from Ocarina of Time, which unlike its counterpart does not burn when hit with fire.
Toon Link holds the Hero's Shield from Wind Waker.
Link being able to crouch to change his shield position is a reminiscent of Zelda II where link is able to shift his shield position to protect his upper or lower body by crouching and standing.