Kokiri are the race of human-like people that live in Kokiri Forest, south of the Lost Woods in The Legend of Zelda: Ocarina of Time. Kokiri never grow up, they are forever young. They live very secretly in the forest, and don't leave because they believe it means certain death. They are watched over and guarded by the Great Deku Tree. The Kokiri "leader" is Mido, a bossy, snobby character that is jealous of Link.
In the beginning of the game, Link lives with the Kokiri in the Kokiri Forest even though he is not one of them. He is not a Kokiri, but when he was a baby, the Great Deku Tree took him in and guarded him after his mother left him there. 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.