The Nihangs are the martial sect of the Sikhs who are devoted to scripture and the Khalsa. 
The Nihangs first appeared in the 18th century as a military element within the Khalsa during the misl period of Maharaja Ranjit Singh. Ranjit Singh recognized there potential and incorporated  Nihangs into his army.  The Nihangs were a fierce and reckless infantry division within the armed forces of Ranjit Singh.  In the earlier period of the Nihangs they were known as the Akalis.

The Nihangs are organized into four armies known as the 'Dal', though all Nihangs feel a sense of common discipline and purpose.  Most Nihangs are unmarried, due to the belief that as soldiers of the Khalsa they must be unencumbered by family ties. For part of the year they remain in their 'camps' (dera) attending to cultivation and cropping.  At other times they roam the Punjab on horseback, conspicuously visibly in their blue garments an for the range of steel weapons which they carry.

The Nihangs are a sect of Khalsa Sikhs, recognizable by their distinctive appearance.  On their head they wear a high turban known as a damala, surmounted by a piece of cloth called a pharhara ('standard' or 'flag').  In their turbans they wear steel quoits, and they carry other weapons such as a cannon-match, two-edged sword, kirpan etc.  Their garments are always blue.  Because they have renounced all dear of death  Nihangs are always ready to die for their faith.  They also renounce worldly concerns, which is what their name, nih-ang means.

Many Nihangs claim that the practice of wearing the high turban and symbolic pharhara was instituted by Guru Gobind Singh.  According to Vivek Singh of Amritsar, however, it was Baba Naina Singh , a military leader of the mid-eighteenth century, who began it.  He did so by attaching his standard to a high turban worn by this standard-bearer, the intention being that the standard bearer should be able to lead with the flag while still having his hands free to use his weapons.

Baba Naina Singh's pupil Akali Pjula Singh became a famous general in the Sikh army of Maharaja Ranjit Singh. Because they are devotees of the Akal Purakh and because they observe the devotional discipline of repeating the word 'Akal' Nihangs are also known as Akalis.

Nihangs are rigorous in their observance of the Rahit.  They live austere lives, distinguished by their love for the scriptures, their fearless acceptance of God's will, and their generosity of spirit.  During the time of the twelve misls and subsequently in the reign of Maharaja Ranjit Singh they rendered notable service as defenders of the Panth and as exemplars of its discipline.

Acknowledgements:
McLeod, W. Hew. Textual Sources For the Study of Sikhism.  Manchester University Press, 1984.