Navratri is celebrated differently in different parts of the country based on local traditions but Goddess Durga remains the main deity. According to history, Mahishasura was bestowed with a boon by Brahma that he will not be killed by anybody except by a female. Upon achieving this boon, Mahishasura started torturing and killing innocent people in order to rule over the whole world and later even tried to rule over heaven. Troubled by this, all devas sought the shelter of the Holy trinity of Brahma, Vishnu and Shiva. The holy trinity combined their primordial powers and created a female form through their shaktis.
This female form was known as Durga and eventually she only could kill the fierce demon Mahishasura. The gruesome war between Goddess Durga and Mahishasura lasted for nine days. On the ninth day, Durga achieved victory over Mahishasura and the 10th day is celebrated as Vijayadasami or Dasara. From the knowledge aspect, Mahishasura represents the inner demons in every being. Ego, hatred, jealousy, enmity, violence etc. are the demonic qualities many human beings carry with them and unless they get rid of these demonic aspects they will not be able to enjoy their lives.