Jihad (Arabic: جهاد ), meaning "to strive" or "to struggle", in Arabic, is an Islamic term and considered a duty by some Muslims. It appears frequently in the Qur'an and common usage as the idiomatic expression "striving in the way of God (al-jihad fi sabil Allah)". A person engaged in jihad is called a mujahid, the plural is mujahideen.

A minority among the Sunni scholars sometimes refer to this Islamic duty as the sixth pillar of Islam, though it occupies no such official status. In Twelver Shi'a Islam, however, Jihad is one of the 10 Practices of the Religion

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The classifications of Jihad are controversial. Dictionaries often define jihad as "Muslim holy war or spiritual struggle against infidels,"[4] but many Muslim sources deny jihad refers to a holy war, while still others define it as combat in the way of God, making no mention of non-violent jihad.

According to scholar John Esposito, Jihad requires Muslims to "struggle in the way of God" or "to struggle to improve one's self and/or society." Jihad is directed against the devil's inducements, aspects of one's own self, or against a visible enemy. The four major categories of jihad that are recognized are Jihad against one's own self (self-perfection), Jihad of the tongue, Jihad of the hand, and Jihad of the sword.
 

Editor Tanveer Zaidi's take on Jihad in Islam According to the Quran and Hadith

1. Jihad in Islam - January 2008