Durga is worshipped in different forms. She is a form of "Shakti". The evolution of Shri Maha Saraswati, Shri Maha Laxmi and Shri Mahakali took place from Shri Brahma, Shri Vishnu and Shri Mahesh respectively. Each of these 3 deities gave rise to 3 more forms and hence in all, these 9 forms together are known as Nav-Durga.