Naruto Uzumaki is a Jinchūriki of the Nine-Tailed Demon Fox. When Naruto taps into the demon fox's chakra, it increases every physical aspect of his body. Increased healing, near-limitless chakra, an almost impenetrable shield, and great stamina. Naruto has unique access to the demon fox's chakra due to the seal the Fourth Hokage placed on him. The seal split the demon fox's chakra in two halves, the Yin chakra made it inaccessible to Naruto, but the Yang chakra allowed Naruto to access it.
By simply tapping into the demon fox's chakra, Naruto's blue eyes turn red, his pupils become slitted, his nails and incisors grow long and sharp, and his whisker marks become more feral. Depending on how much chakra is unleashed, the red chakra could glow around his body in an untamed way. Naruto first reached this form when Sasuke Uchiha was thought to be killed by Haku, causing the pure rage to take over allowing the demon fox's chakra to leak out of the seal.
Even though this is the weakest state of his Jinchūriki forms, it is still a deadly foe as it increases his physical strength and healing well above most shinobi, and increasing his speed to such high levels that nothing short of a fully matured Sharingan is able to track his movements. Until special training from Jiraiya, Naruto could only reach this state through rage or if his life was in imminent danger, but after having a inner talk with the demon fox, he is able to tap into the demon fox's chakra on his own, or by simply telling the demon fox to give him some power.
 One-Tailed Transformation
During Naruto's battle with Sasuke at the Valley of the End, when Naruto was at the mercy of Sasuke, the demon fox told Naruto that he should be grateful to it and gave him more chakra. The amount of chakra is so great that it leaks out of his body in a denser state. All the physical traits of the basic Jinchūriki form are present and slightly altered, including a black ring appearing around his eyes, longer nails and canines, and more beserk looking eyes, with one major bonus; the red chakra becomes a complete shield around his body, in the shape of a fox, complete with long ears and a tail, known as the demon fox's cloak or shroud.
In his one-tailed transformation, Naruto fights more like a beast. Instead of using his fists, he instead uses his sharpened claws. While he can stand on his own two feet, he can move at greater speeds on all fours. But his most powerful weapon is the chakra itself. The bubbling chakra appears to have a mind of its own: when a strike that was meant for Sasuke missed, the chakra itself stretched off Naruto's arm and hit Sasuke before he could react. Since the chakra has a mind of its own, it is extremely difficult, if not impossible, to predict Naruto's movements, as Sasuke was able to see Naruto perfectly, but couldn't predict what the chakra would do with the Sharingan. The chakra can stretch far from the body, as Naruto was shown using his "arms" to attack Sasuke from great distances, at one point being on an opposite side of the valley, and still attacking him with the chakra arms. The shield around his body appears to be impenetrable, comparable to Gaara's Ultimate Absolute Defense: Shield of Shukaku.
 Two-Tailed Transformation
After catching the Akatsuki member, Deidara, and beating what he thought was Deidara to pieces only to realize he had been attacking a substitution, his anger at losing Deidara and not being able to avenge Gaara's death caused Naruto to slip into his two-tailed transformtion. His physical characteristics match the first two forms, but his red eyes and incisors grow larger, and his lips get a black outline, similar to fox gums. The chakra shield's only difference between the one-tailed transformation and the two-tailed is the extra chakra tail that has grown.
While in this state, Naruto is still in control of his body, though to what extent is unknown. In the anime, he punched a tree, destroying it and a long line of trees behind it, and later he attempted to attack Kakashi Hatake, who later tried to calm him down, and possibly tried to stop him from finding Deidara. The amount of chakra produced by the two-tailed transformation was so great, that even Naruto's shadow clones doubled over in pain from the chakra.
 Three-Tailed Transformation
After Orochimaru revealed himself to Team Yamato and baited Naruto with news of Sasuke, Naruto went on a rage and first attacked him in his one-tailed state. When that failed, along with further baiting from Orochimaru, Naruto skipped the two-tailed transformation all together, and went straight into his three-tailed transformation. The three-tailed state looks exactly like the two-tailed, again the only difference being the number of tails. His eyes become similar to the four-tailed transformation, but can return to normal.
The amount of chakra produced from this form is devastating. The very air around Naruto begins to hurt. This is the strongest form Naruto can reach and control. With the standard chakra shield's protection and attack power, a simple roar has the power to create a devastating sphere of destruction.
 Four-Tailed Transformation
When Orochimaru was fighting the three-tailed transformation, Naruto turned to the demon fox in desperation and anger, and slipped into the demon fox's grasp. The demon fox's chakra began to burn its way through Naruto's body, destroying his skin, and rapidly healing it at the same time, eventually covering his entire body in a mix between the demon fox's chakra and his own blood. Everything from his face to his chakra tails became completely solid, his eyes losing its characteristics and became empty glowing circles surrounded by a black mask made of chakra.
In this state, in effect, Naruto takes the place of the demon fox inside the cage. The demon fox has complete control over Naruto's body, making it a mere skeleton for the fox's chakra, and every living thing in its way a target. The four-tailed transformation has all the benefits of the other tails, with upgrades. With a simple wave of his hand, it can send a shockwave of destruction for miles. The chakra shield, while being solid, still completely protects Naruto's body, and is so hot that it can burn the skin on contact. If Naruto is hit, the chakra itself can form another body, and strike back almost instantly. The demon fox also has the ability to charge a concentrated ball of chakra, consume it, and then fire it with such a force that it could destroy three Rashomon gates without losing strength. Finally its chakra shield's defensive power is so great that it is impregnable to any form of attack. Even Orochimaru's legendary Kusanagi couldn't even break the exterior shell of the demon fox's chakra, only bending it slightly.
In the end, despite it being Naruto's strongest Jinchūriki form revealed so far, the four-tailed transformation is Naruto's greatest downfall. Because the demon fox's chakra is constantly destroying and creating new cells, it also shortens Naruto's life, similar to Tsunade's Creation Rebirth. Also, because the demon fox has complete control, Naruto will attack anything that moves, including his friends. This discourages Naruto from using this state more than the fact that it shortens his life. Naruto first slipped into this state when he was training with Jiraiya, because he used the toadkey to "twist" the seal the Fourth Hokage placed on Naruto. A small "twist" of the seal unleashed the four-tailed transformation and almost killed Jiraiya.
 Jinchūriki Chakra Control
In the event of the demon fox manifesting, Naruto's sensei, Jiraiya, gave Kakashi a special written seal that is to be placed on Naruto's body. This seal suppresses the demon fox's chakra and restores Naruto to his normal self. In addition, Yamato has demonstrated a technique that reverses Naruto's demon fox transformation, enhanced greatly by the First Hokage's necklace that Naruto wears. Naruto, in the anime, has shown that with enough will power, he is able to suppress the demon fox's chakra from taking over. However, the important factor to this is that the demon fox's chakra was responding to the vessel of its residual chakra so it is unknown if Naruto could use his willpower to suppress the demon fox's chakra if it forced its way in a similar fashion as his first four-tailed transformation.
Sasuke has also demonstrated the ability to forcefully suppress the demon fox's chakra. To parallel the demon fox's apparent connection with the Uchihas, Itachi Uchiha of Akatsuki was originally assigned to capture it as part of the organization's goal of obtaining all the Tailed Beasts. This task was later given to Pein.