Akhara means literally the place for practice (Greek word academy has similar meaning, and is in fact cognate) for the protection of Sanatana Dharma. Akhara is divided in 8 davas (divisions) and 52 marhis (centers). Each Marhi is governed by a Mahant. The top administrative body of the Akhara is Shree Panch (the body of five), representing Brahma, Vishnu, Shiva, Shakti and Ganesha. It is elected on every Kumbh Mela and the body holds its post for 4 years.
Akharas are divided into different types according to the concept of God they worship. Shaiva Akharas for followers of Lord Shiva, Vaishnava or Vairagi Akhara for followers of Lord Vishnu and Kalpwasis for followers of Lord Brahma.
In the beginning Adi Shankaracharya established 7 Akharas namely Mahanirvani, Niranjani, Juna, Atal, Avahan, Agni and Anand Akhara. Today there are 3 major Akharas (Mahanirvani, Niranjani, Juna) and 3 minor Akharas (Atal affiliated with Mahanirvani, Anand affiliated with Niranjani, Avahan affiliated with Juna). Furthermore there is one small Brahmachari Akhara named Agni, affiliated to Juna.
The first group is known as Naga Sannyasis; their initiation ceremony takes place only during Kumbh Mela. Only those who were initiated during Kumbh Mela in Prayag are eligible to be Shree Mahants. The initiation ceremony for nagas are different from those for the second group (Paramahansas and Dandis).
The biggest Akhara - regarding the number of the Sadhus in it - is Juna, then Niranjani and then Mahanirvani. The first person in the Akhara is the Acharya Mahamandaleshwar, followed by other Mahamandaleshwaras, Mandaleshwaras and Shree Mahants.