Inside the Deku Tree is the first dungeon in The Legend of Zelda: Ocarina of Time. Link enters the Great Deku Tree in order to dispel a curse placed upon him by Ganondorf that is slowly killing him. The source of the curse turns out to be a parasitic monster attacking the tree's roots. Using his newly acquired slingshot, Link is able to slay the parasite and break the curse, but is too late to save the Deku Tree who gives him the first spiritual stone (the Kokiri Emerald) before passing away.
The main item in this dungeon is the slingshot. A map and compass can be found as well.
Queen Gohma, a giant gohma, resides here. She has many minions she will order to attack you, and she will climb on the ceiling and attack you from above. Her weakness is her eye, which you hit with a pellet from your slingshot and then attack her with your sword. It is a good idea to use deku nuts in this battle, because they stun her for a short time, giving you ample time to attack with sword jumps for extra damage.