Silver Arrow
The Silver Arrow is a special upgrade to the Bow & Arrows used to defeat Ganon. These arrows appeared in both the first Legend of Zelda and A Link to the Past. In later games of the series, the role of these arrows, as both powerful weapons and items needed to defeat the final boss, was replaced with Light Arrows.
The Legend of Zelda
In Legend of Zelda, the Silver Arrows are found in Level 9, the final dungeon. After hitting Ganon with his sword, Link used the Silver Arrow to finish him off. Besides being able to finish off Ganon, these arrows work exactly the same as regular arrows against every other monster.
A Link to the Past
In A Link to the Past, Silver Arrows are given to Link by the Fat Fairy inside the Pyramid of Power, once the way has been opened with a Super Bomb. These are more powerful versions of the typical arrows Link obtains, and are arguably some of the most potent weapons available. When used as weapons in the field, they can kill most enemies in a single shot, including some bosses. Once again, they are used to kill Ganon, or the more technical term, "Give Ganon his last".
The Ancient Stone Tablets
In The Ancient Stone Tablets, the Silver Arrows appear from a giant stone slab that sealed away Ganon's Tower from the world.