Small Keys are Dungeon items featured in every The Legend of Zelda game to date. They are used to open locked doors or locked blocks, as seen in Link's Awakening. These locked doors and blocks are only found in dungeons. They can usually be obtained by solving a puzzle or defeating a whole bunch of enemies, which makes a small Treasure Chest appear, and usually contains a Small Key. They do not open the door to boss rooms. For said doors, Link needs the Boss Key or Big Key.
In every game except for The Legend of Zelda, the only place to acquire a Key is in a dungeon. However, in The Legend of Zelda, some Item Shops that are found around Hyrule sell Keys for 100 Rupees a piece. Also, The Legend of Zelda is the only game in the series where a Key found inside a dungeon can be taken out and used in another dungeon. This feature is not present in any other game, as in them a Key can only be used in the dungeon it is found in.
Small Key Official Artwork from The Legend of Zelda
Small Key Million Publications Strategy Guide artwork from The Legend of Zelda
Sprite from The Adventure of Link
Sprite from A Link to the Past
Small Key Sprite from Link's Awakening (Game Boy)
Small Key model from Ocarina of Time
Small Key model from The Wind Waker
Small Key Sprite from The Minish Cap
Small Key from Twilight Princess
Small Key from Phantom Hourglass
Small Key icon from Skyward Sword
Small key from A Link Between Worlds
Small Key from Breath of the Wild