Kokiri are a recurring race in The Legend of Zelda series. First appearing in The Legend of Zelda: Ocarina of Time, they are diminutive, childlike forest sprites that live in Kokiri Forest, south of the Lost Woods. Kokiri resemble Hylian children only they never grow up, they are forever young. Each Kokiri is bonded to a Fairy partner that lives with them and helps them communicate and find their way in the forest.
The Kokiri live very secretly in the forest, and never leave because they believe it means certain death. They are watched over and guarded by their father, the Great Deku Tree who is the guardian deity of the forest. The Kokiri "leader" is Mido, a bossy, snobby character that is jealous of Link.
At the beginning of Ocarina of Time, Link lives in Kokiri Forest and believes he is a Kokiri, however unlike the rest of the Kokiri he has yet to receive a fairy of his own. After receiving the Master Sword and becoming an adult the Deku Tree reveals that Link is Hylian and not a Kokiri, and that when he was a baby, the Great Deku Tree took him in and adopted him after his mother died fleeing from a civil war. He is a Hylian, but he is accepted among all the Kokiri except for Mido. Saria, a very important character and Kokiri, is Link's best friend, and she helps often throughout the game.
Although unconfirmed, it is very likely that the Koroks from The Legend of Zelda: The Wind Waker are evolved forms of the Kokiri. The Great Deku Tree (a different one) guards them, and talks about how they used to have human form, but transformed when the land flooded.