In the Naruto universe, nine immensely powerful demons known as the tailed beasts exist, each distinguished by the number of tails it possesses. These creatures are beings of pure living energy, or chakra, and can easily overcome most ninjas. The drawback to their power is that the tailed beasts are extremely instinctual creatures and are unable to utilize their skills fully.
Ninja villages seeking to harness the power of a tailed beast are able to seal it into a human's spirit. The host, or Jinchuuriki, of a tailed beast can then utilize this power against his village's enemies.
In the beginning of the original Naruto series, the Fourth Hokage, leader of the Hidden Leaf Village ninjas, sacrificed his life to seal the attacking nine-tailed fox inside the infant Naruto, making him a Jinchuuriki.
The nine-tailed fox demon is the most violent and powerful of all the tailed demons, giving Naruto the ability to access skills that would normally be outside the range of the ninja within his age bracket.
The nine-tailed fox is able to manifest itself physically as Naruto attempts to draw more power from it, or if he becomes emotionally overrun during battle. As the nine-tailed demon appears, Naruto will slowly take on more characteristics of the fox, most notably tails. The number of tails emerging from Naruto is the best way to gauge the amount of control the nine-tailed fox is exerting over Naruto.
Naruto has only been seen with a maximum of four tails in the series. When four tails manifest, the ninja loses complete control of his actions and the fox demon takes control. At this point, Naruto appears more beast-like than human and has no recollection of events that transpire during his transformation.
In Naruto Shippuden episode 39, "The Tenchi Bridge," Orochimaru appears along with his subordinate, Kabuto, to meet team Kakashi face-to-face. Naruto still holds Orochimaru responsible for the departure of his former team member Sasuke and demands that he be returned. Orochimaru goads Naruto into a fight by suggesting that Naruto should force the information from him.
Over the course of the following episode four of the nine-tailed fox's tails emerge from Naruto during his battle with Orochimaru which continues through episode 40 and concludes in episode 42.