Goddess Durga is worshipped in different forms . She is a form of " Shakti" .The evolution of Shri MahaSaraswati, Shri Mahalaxshmi & Shri Mahakali ( the main 3 forms of Shakti) took place from Shri Brahma , Shri Vishnu & Shri Mahesh respectively.Each of these 3 deities gave rise to 2 more forms.So , in all , these 9 forms together are known as " Navadurga ".
Long back there lived a daemon king named " Mahishasur " ( also called as Daemon with a face of buffalo) who was very strong.He harrassed God Indra , fought with him and captured his throne.He was very proud of his strength.Indra went to the almighty gods - Brahma , Vishnu , & Mahesh , for help.These gods decided to teach a lesson to Mahishasur.They unitedly created a goddess "Shakti" and offered her their weapons to fight Mahishasur, who had become untolerable.Goddess Shakti fought with Mahishasur for full 9 days and ultimately killed him with her spear.Hence Durga is also known as " Mahishasur-Mardini ".
The 9 days for which the goddess fought & Killed the evil daemon Mahishasur are celebrated as a major festival called " Navaratra".This festival is celebrated in a big way all over India in the month of " Aashwin" ( as per Hindu Calender)for the first 9 days .
Names of Durga :
The goddess Durga is called by various names such as Uma , Gauri , Parvati , Kali , Chandi , Bhairavi, Chamundi , Jagadamba , Aadimaaya , & Tripursundari.
9 NAMES OF NAVADURGA :
Rudrachandra , Prachanda , Chandogra , Chandnayika , Chanda , Chandavati , Chandarupa , Atichandika , Ugrachandika .
Mahalaxshmi , Nanda , Shemkari , Shivduti , Mahamdra , Brahmari , Chandamangala , Revati & Harsiddhi.
Rudranshdurga , Vandurga , Agnidurga , Ripumaridurga , NilKanthidurga , Shemkaridurga , Vindhyavasinidurga , Harsiddhidurga.