This beast gives Naruto the ability to heal from wounds at astonishing speeds, as well as give him an immense amount of
chakra. It is also the reason why Naruto has those three lines (or whiskers) on each side of his face. The
Nine-Tailed Fox is activated in times of extreme rage (like when Naruto saw Sasuke take a hit for him and thought that he was going to die) and times of near death (like when Naruto is about to drown after being heavily wounded in the battle between him and
Sasuke). As the Nine-Tailed fox is activated, Naruto becomes more and more like the
Nine-Tailed Fox. The Third Hokage and Kakashi Hatake have said that if Naruto loses all control of the
Nine-Tailed Fox’s chakra, than the seal which keeps it inside Naruto will break and the
Nine-Tailed Fox will be freed.

If the Nine-Tail Fox escapes from Naruto, then we will die. Fortunately
for Naruto and his comrades this has not happened yet. He has been trained by one of the three legendary
Sannin, Jiraiya, so that he could control and call upon the power of the
Nine-Tailed Fox at will.

This is the first form Naruto has taken under the influence of
the Nine-Tailed Fox. Naruto’s eyes turn a ferocious red, his whiskers
increase in size, his hair becomes a little more animal like, his
fingernails and teeth grow sharp, and he runs on all four of his limbs.
In addition, Naruto gets an energy boost from the Demon Fox’s chakra,
healing his wounds, increasing his speed, and giving an extra bang to
his attacks. Naruto is conscious at this level and is able to control
and summon this stage at will after receiving training from Jiraiya.

When it seemed like it was the end for Naruto in his fight against Sasuke
in the Valley of the End, the Nine-Tailed fox grants Naruto more chakra
than he has ever received before. In this stage Naruto gains all the
physical attributes that he gained in the first stage as well as a
1-Tailed Fox outline around his body made of chakra. This chakra is hot.
So hot, that the water around Naruto is boiling as Naruto is in this
stage. The fox-shaped chakra around Naruto’s body isn’t just for show
though. It increases Naruto’s reach, as he can thrust his physical arm forward
and attack his opponent with an extendable arm made of chakra. In addition
to that, it can act as a defense mechanism and deflect incoming attacks
without Naruto even controlling it. And of course, Naruto can heal quicker
than normal, and is faster and stronger than he was in his tail-less Demon
Fox form. Despite his beastly look, Naruto is conscious at this level, and
can control what he does.

This is Naruto’s third Demon Fox stage. In this stage he inherits all
the traits that he had before, but now instead of one-tail, he has two
tails instead. In addition, Naruto looks more monstrous than before,
with his eyes widening, and teeth becoming even more sharper. As to be
expected, he is faster and stronger than the previous stage. Like
before, he is still conscious, but it begins to look like the Demon
Fox’s personality is more visible than before.

This is Naruto’s fourth Demon Fox stage. In this stage he inherits all the
physical traits that he had in the previous stage, but now instead of
two-tails, he has three. Naruto can now counter attacks and do damage just
by simply roaring, indicating he is more powerful than he was at his
previous states.

This is Naruto at the 4-tailed stage. Unlike the first four stages where
Naruto was in control of himself, in this form, the Nine-Tailed Fox has
a grasp of Naruto in his internal Demon Fox prison. This result is that
Naruto has no control of his body and the seal that traps the
Nine-Tailed Fox in Naruto loosens further. In addition, the chakra that
the Demon Fox emits overtakes Naruto’s physical body, leaving no
visibility of human flesh. He is essentially on fire with chakra with
his physical features are similar to that of the Nine-Tailed Fox
himself. Not only does Naruto’s appearance change, but his power is at
its greatest ever. Just a roar produces frighteningly powerful and
destructive effects. Even Orochimaru says that if he is to get hit by
Naruto at this stage, he’ll get killed for sure. Unfortunately for
Naruto, this stage causes Naruto’s lifespan to shorten. Being at this
stage destroys his cells, but at the same time, because of the
Nine-Tailed Fox’s special ability, generates new cells o replace the
destroyed ones. It would seem it would be a good thing for new cells to
be made right away to replace the destroyed ones, but it is not.
Everybody’s body has a limit to how many cells can be produced in a life
time. It is encoded in the genetic code. Because cells are being
regenerated at an accelerated rate in Naruto’s body so quickly, his life
span is decreasing since he won’t have the ability to produce new ones
at some point later in time. (This is not only true for Naruto, but for
us humans in real life. Cool huh? Who knew NARUTO was so educational)