Etymologically the word Roh is thought to be derived from the Sanskrit Rohitagiri ( ‘ Red Hill ‘ ). Panini , Sanskrit grammarian , who lived most probably in the fourth century B.C. , informs that Highlanders dwelt on the Rohitagiri. The word Rohitagiri assumed the terminology of Rohi with passage of time and was known as such by 400 A.D. when Fa-hian , the Chinese traveller , travelled through present-day Afghanistan. 
Khulasat-ul-Ansab described Roh as “the large tract of country belonging to, and inhabited by, the Afghans, the eastern boundary of which extends to Kashmir, and the western to the river Hirmand, a distance of two-and-a-half months’ journey; and on the north its boundary extends to Kashkar (Chitral), and its southern boundary to Baluchistan. It therefore lies between Iran, Turan, and Hind. “
Those who migrated from Roh (Pakhtunkhwa) to India in 16th, 17th and 18th century, were designated as Rohela (meaning inhabitant of Roh) by Indians. Pashtuns in their native lands do not call themselves Rohela (روهیله) , the suffix -ela is Indian nomenclature (for example Bundela, Baghela etc).
The real founders of Rohilla power in India were an Afghan adventurer, named Daud Khan, who arrived in India immediately after the death of Aurangzeb, and his adopted son, ‘Ali Muhammad Khan, who succeeded him as leader of a band of mercenary troops. Ali Muahmmmad Khan carved out for himself a principality in Ketehr (U.P, india). It was during lifetime of Ali Muhammad Khan that Katehr came to be known as Rohilkhand. Rohilla rule was terminated in 1774 when the country was overrun by British. Rampur was the only Rohilla principality which survived as a princely state under British rule. Rampur state was created by Faizullah Khan, a son of Ali Muhammad Khan.
As late as 1815, Pashto was spoken in Rampur. Mountstuart Elphinstone writes in 1815 :-
“The residence of the Nabob (Nawab) is at Rampur, the manner of which place still resemble those of the Berdooraunees (Bar-Durranis). Pushtoo is the principle language, and one sees in the square before the Nabob’s place, fair, strong, and handsome young men, sitting or lounging on beds, with that air of idleness and independence which distinguishes the Eusofzyes (Yousafzais).” .
Rohillas maintained a remarkable equality among themselves whenever they assembled. It was difficult to distinguish between a sardar and an ordinary soldier in their assembly. For a long time they also maintained their traditional tribal dress of their old country which consisted of long and loose kameez and tumbans (loose shalwar) of coarse cloth. They used broad long pieces of blue-colored cloth tied at the waist with large rumals, or handkerchiefs, thrown over the shoulder. Turbans were indispensable part of their dress which distinguished them from the Mughals by its ‘oblique setting. 
Rohillas were famous for their tolerance towards their Hindu subjects. Mountstuart Elphinstone writes, “Rohillas are the bravest soldiers we have ever contended with in India. Their kindness to their Hindoo subjects cannot be denied” . Najib-ud-Daulah did not approve Maulvi Nazar Muhammad act of forced conversion of a Hindu Khatri woman to Islam. As administrator of Delhi, Najib-ud-Daulah expelled the Maulvi from the city. At Hapur, just before his death, Najib-ud-Daulah learnt that the Hindu festival at Garh Mukteswar was in progress, and instructed his followers to ensure that none of the pilgrims were subjected to harassment or plunder . Hafiz Rahmat Khan and his sardars supported Hindu scholars and priests, granting them financial assistance for their subsistence. 
Rohilkhand greatly flourished under the rule of Rohillas. Mountstuart Elphinstone writes;
“The state of improvement to which they [Rohillas] had brought their country, excited the admiration of our troops, and has been displayed with enthusiastic eloquence by Burke. The coldest phrases express that “it was cultivated like a garden, without one neglected spot in it.”Even now it is among the richest parts of the British provinces. It consists of vast plains, covered with fields of corn, or orchards of Mango trees, and filled with populous towns and flourishing villages.”