Keys are items found in the dungeons of every Zelda game. They are necessary for advancing through locked doors. Keys can be obtained through a variety of different puzzles. For example, sometimes you may need to defeat all the enemies in a room. Other times you may need to break a hole in the wall with your bombs. In some situations you need to do what you least expect, and that way it will be easier to advance to victory. Once a key is used, it disappears.
There are roughly four different keys:
Small Keys are the most common keys.
Boss Keys are big keys that grant access to the dungeon's boss.
Dungeon Keys only appeared in Link's Awakening, Oracle of Seasons and Oracle of Ages and were needed to enter their respective dungeons.
Phantom Hourglass featured three keys needed to open doors spread across the islands of the Great Sea!!!