Fairies are a common species in The Legend of Zelda series. They are also sometimes called Pixies and they usually restore health or provide an extra life if Link loses all his hearts. They originated in the first The Legend of Zelda. A Great Fairy can heal Link, give him an extra power, or upgrade his items.
The Legend of Zelda
In The Legend of Zelda, fairies simply heal Link whenever he finds them. They are indistinguishable from Great Fairies, with the exception of Great Fairies being found by ponds.
Ocarina of Time
In The Legend of Zelda: Ocarina of Time, fairies either heal Link or provide an extra life. They can be kept in a bottle for future use as a healing item or a revival from death.
Four Swords Adventures
In The Legend of Zelda: Four Swords Adventures, Force Fairies provide extra lives and are acquired throughout the game, including upon the completion of a level.