The monastic structure of Sanatana Dharma is complex and organic, most monks belong to a parampara (lineage) or a monastic order. The Akhara vary in theology as well as size and influence; most are Smarta in practice, but three are considered Vaishnavite and a few are Saivite. There is plenty of respectful acceptances for different points of view among the groups and no dominance exists. Other large exist outside the Akharas, such as the Swaminarayan fellowship, the RamaKrishna Order and the Nathas of South India. Within each Akhara, most sannyasins are also associated with one of the dashanami lineages. Adi Shankara, who originated the dashanami system and did much to revitalize and improve the system of monastic orders.
Sadhus are in two broad categories, those who love in monastic communities and those who wander or live in solitude. The socially engaged Juna Akhara (The Juna Akhara is the largest of the thirteen akharas consisting of over 500,000 sadhus and five million sanyassis) consists of both types and is host to thousands of naga sadhus and maunis who strive to uplift the world by their silent existence, anonymously blessing society from secluded caves. Their lack of involvement maintains certain fluidity in the monastic order; these outlying branches of the family prevent the Acharya from becoming too central to the structure. During the monumental Kumbha Melas, the mountain dwelling Sadhus join their brothers of the same Akhara in a surprisingly harmonious weave, populating vast areas under colorful tents and new sannyasins are initiated by the Acharaya Mahamandaleshwar, usually by the thousands in a powerful event.  

Literally, "Mahamandaleshwar" means "Superior of great and/or numerous monasteries" or "Superior of a religious district or province" (maha = great, mandala = district, ishwara = head, sovereign). There are two systems of organization in the dashanami order. Each of the ten names (dasha = ten, nami = name), which is appended to a monk's given name at initiation, is assigned to one of the four cardinal monastic seats founded by Shankaracharya and presided over by his direct successors at Joshimath in North India, Dwarka in the West, Shringeri in the South and Puri in the East.

The other system is the akhara system of which the Mahamandaleshwaras are a part. Their origin dates from the early days of Islamic invasions into India (around or just prior to 1000 CE) when many monks became the targets of murderous attacks. In order to protect themselves, they formed "regiments" of monk-warriors who, to this day, preserve the traditional martial arts of India in addition to their spiritual practices. So in addition to being a way to organize the orders at a local level they have historically also served as paramilitary regiments with the Mahamandaleshwaras as generals, as it were, of these regiments.

Mahamandaleshwar Swami Kumaranand Saraswati Ji Maharaj

Acharya Mahamandaleshwar Yugpurush Swami Paramanand Ji Maharaj

Barbara Marx Hubbard

Mahamandaleshwar Swami Nityananda

Mahamandaleshwar Swami Vishnu Devananda Giri Maharaj

Mahamandaleshwar H H SriSwami Satyamitranandaji Maharaj

Mahamandaleshwar Shaktiananda Ma Lattin America

Mahamandaleshwar Swami Shankarananda

Mahamandaleshwar Swami Swarupananda Western Church of Sanatana Dharma founded in 2008

Michael Murphy

Rothko Chapel

Source:
http://en.wikipedia.org/wiki/Mahamandaleshwar