About the Tails

Unleashed Bijuu Power Form
The first Kyuubi form revealed in the series, this form grants Naruto some measure of Kyuubi's chakra. This form typically emerges when Naruto becomes enraged. His whiskers turn more feral and his eyes turn red. His hair becomes bushy, his incisors grow longer and his fingernails grow sharp. In this form he is surrounded in the Kyuubi chakra and is able to move more quickly, use more chakra draining jutsu, heal his body faster and strike with more force.
Jinchuuriki-Bijuu Form (One-Tail)
When Naruto fought Sasuke at the Valley of the End, he was desperate to get his friend to return to Konoha. The fight pushed Naruto past his limits. When the fight began to turn in Sasuke's favor, the Kyuubi granted Naruto even more power than before, causing the red chakra surrounding Naruto to boil and bubble and take on the shape of a fox with long ears and a tail. This chakra is not bound to Naruto's body, being able to move and extend in as striking limbs. Though Naruto remains conscious in this state, the chakra can seemingly react of its own will. In this state the chakra will also act as a defense against incoming attacks.
Jinchuuriki-Bijuu Form (Two-Tail)
When entering the two-tail state, Naruto becomes even more beast-like in appearance. His eyes grow larger and become outlined in a rough black. His incisor teeth also grow even larger than before. In a curious extension between the original Naruto and his shadow clones, when he entered this state his clones began to feel an intense burning within them, causing them to double over in pain.
Jinchuuriki-Bijuu Form (Three-Tail)
Naruto's three-tail form resembles that of two-tails. In this state Naruto was able to roar and extend chakra outwards to devastating result. This outwards destructive sphere of chakra was so intense so as to destroy surrounding matter and violently push back any incoming attack with but a simple roar.
Jinchuuriki-Bijuu Form (Four-Tail)
In this form Naruto loses all sense of himself, becoming a pure destructive beast with no regard for even his closest friends. In Naruto's internal Kyuubi prison, he crosses the protective barrier and falls into Kyuubi's grasp. Externally the Kyuubi chakra begins to melt off Naruto's very skin. His body changes to resemble the bushy whiskers on his face when enraged, and his hands form sharp claws. Ears and four tails extend from his body and he roars with a sharp maw of teeth. This form proves insanely destructive, able to roar, swipe and attack with hugely destructive results. And should any foes dare touch his form, they'll be burned by the intense heat. Unfortunately such prolonged usage of the Kyuubi tailed state is further weakening the seal Yondaime Hokage used to contain the bijuu.