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.