Mujahideen (Arabic: مجاهد muǧāhid, nominative plural مجاهدون muǧāhidūn, oblique plural مجاهدين muǧāhidīn "strugglers" or "people doing jihad") is a term that Muslims use to describe those they see as Muslims who struggle in the path of Allah. The word is from the same Arabic triliteral root as jihad ("struggle"). In recent years, Mujahideen has been most closely associated by the west with radical Islam, encompassing several militant groups and struggles.
The beginnings of Jihad are traced back to the words and actions of Mohammad and the Qur'an. The people who helped Mohammad were referred to as Ansars ("helpers") andMuhajirs ("immigrants" who left due to years of persecution in Makkah, settling in Madinah). Then the Muhajireen's property was confiscated in Makkah, so Mohammed called upon the Muslims to participate in Jihad against the Quraysh.
The earliest known expeditions they participated in were the Caravan
raids, where they were given the task of intercepting Quraysh caravans.
They also participated in other battles, such as the Battle of
Badr and Uhud.
The term Mujahideen was first used by the West to describe the mountainous sect of hillmen in Afghanistan who fought against British control (although initially to the British they were known as Sitana Fanatics). It began in 1829 when a religious man, Sayyid Ahmed Shah Brelwi, came back to the village of Sitana from a pilgrimage to Mecca and began preaching war against the infidels in the area defining the Northwest border of British India. Although he died in battle, the sect he had created survived and the Mujahideen gained more power and prominence. During the Indian Mutiny of 1857, the Mujahideen were said to accept any fleeing Sepoys and recruit them into their ranks. As time went by the sect grew ever larger until it was raiding and controlling larger areas in Afghanistan.
courtesy : http://en.wikipedia.org/wiki/Mujahid