The Great Deku Tree was the guardian protector of the Kokiri in The Legend of Zelda: Ocarina of Time. Young Link ventures inside the tree, and finds a huge monster within him.
The monster was Gohma, who had been slowly killing the tree. After Link defeats Gohma, the tree dies but not before passing on knowledge vital to Link's quest and a nice gemstone.
In Hyrule's future world, The Great Deku Tree body remains. However a little sprout formed and could actually talk.
Fans figure the little sprout grew up to be the Great Deku Tree in The Legend of Zelda: The Wind Waker.