What is Kurama or Kyuubi ??

Kurama first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tails' resurgence, the Sage split up its chakra and used his Creation of All Things (万物創造, Banbutsu Sōzō) ability to create nine separate constructs of living chakra, which would become known as the tailed beasts. Some time after being created, the Sage sat down with all the young tailed beasts and told them that they would always be together even when separated, and that one day they would become one entity again with different names as well as forms than they did then when the time came for them to understand what true power is.
Over the centuries, Kurama has gained a reputation as an age-old natural disaster, appearing suddenly out of nowhere to attack areas wherever human malice had collected and festered.

At one point during the time of the formation of the shinobi villages, the Gold and Silver Brothers of Kumogakure were assigned to capture Kurama, but they were both swallowed whole by the fox. However, the two brothers managed to survive and gain some of its power by eating the flesh of its stomach for two weeks, which caused Kurama to regurgitate them.

The battle between Madara and Hashirama, with Kurama under Madara's control.
After that event, when Madara Uchiha defected from Konoha, he used his Sharingan to control and use Kurama to help him fight against Hashirama Senju, the First Hokage, to exact his revenge. The battle was so great, it created a vast crater that later became known as the Valley of the End. Despite being in control of Kurama, Madara was defeated and was believed to have died.[9] After the fight, Hashirama's wife, Mito Uzumaki, sealed Kurama within herself, becoming its first jinchūriki. Towards the end of her life, Mito passed on this status and responsibility to another member of her clan, Kushina, who later became the wife of Minato Namikaze, the Fourth Hokage.

Kurama destroying Konoha.
Twelve years before the start of the series, after the end of the Third Shinobi World War, the seal containing Kurama within Kushina weakened as she became pregnant and gave birth to Naruto. Despite keeping her childbirth a secret, Tobi discovered their location, killed their escorts, broke the seal, and seized control of Kurama. Since Kushina survived the extraction, Tobi tried to have Kurama kill her, but Minato arrived in time to save Kushina and brought her to safety with Naruto. After that, Tobi summoned Kurama into Konoha, and ordered it to destroy the village.
While Kurama was rampaging in the village and decimating the shinobi forces that tried to repel it, Minato, in his battle with Tobi, placed a Contract Seal on the masked man, which immediately freed the fox from his control. Before Kurama could annihilate Konoha, Minato summoned Gamabunta on top of the fox, and later teleported it to Kushina and Naruto's location, while letting its Tailed Beast Ball detonate away from the village. Soon afterwards, Kushina used her chakra chains to subdue Kurama, planning to seal it back inside her before she dies. However, Minato, knowing that Kurama would revive without a host and a threat Tobi still was to the village, came up with a different plan to give Konoha the means to combat Tobi if he should ever attack the village again.

Kurama, after having its Yin chakra sealed away.
Since Minato himself couldn't fully seal Kurama as it was, he used the Dead Demon Consuming Seal to seal its Yin chakra within the death god, before preparing the Eight Trigrams Seal to imprison the fox, along with its Yang chakra, within Naruto.[14] Kurama realised Minato's intention and took the chance to attack Naruto when Kushina was weakened, but its attack was unsuccessful as both Minato and Kushina quickly used their own bodies to shield their child, piercing them instead. Before the sealing was complete, Kurama used its last moment of freedom to curse the pair.
In the anime, it was revealed that Kurama had left behind large amounts of its chakra during the attack. Kazuma collected and sealed the chakra within his own son, Sora, hoping to make use of its power for his own agenda.