The Hylian Shield is a recurring shield in The Legend of Zelda series. It is Link's most iconic shield, being wielded by him in most official artwork and is usually one of the better shields in the games where it appears. It's first named appearance was in The Legend of Zelda: Ocarina of Time, however, Link's unnamed starting shield from The Legend of Zelda: A Link to the Past bears a strong resemblance to it in official artwork.
The Hylian Shield is a blue, iron, heater shield with a wooden backing. The shield's crest depicts a red Loftwing flying towards the Triforce. Because it is made primarily of iron, the Hylian Shield won't burn up when exposed to fire unlike the wooden shields that Link tends get at the start of his adventures. It's durability and ease of use makes the Hylian Shield is very popular with the Hylian knights it was primarily designed for and the Bazaar in Hyrule Castle Town even offers a word of mouth discount for soldiers allied with the Royal Family.
In Ocarina of Time, it's actually not the most powerful shield, with that title going to the Mirror Shield, which is better primarily for its ability to reflect light. It has been said that the shield is so powerful due to magic that surrounds it, though this cannot be confirmed.