Islam / Ridda / Takfir /
Arabic: kāfir (sing.) kuffār or kāfirūn or kafara (pl.)
Depending on definition, a kafir can by any non-Muslim, or anyone not belonging to the Ahl al-Kitab (Muslims, Christians, Jews, Sabaeans or, possibly, Zoroastrian). In the latter case, anyone from the Buddhists, Hindus to Atheists are kafir.
A kafir is by definition not a person with another opinion than Islam, he or she is simply one who refuses the self-evident truth. His or her opinion is therefore only recognized for its lack of truthfulness, not worthy the attention of a Muslim.
In actual Muslim theology, there appears distinctions, ranging from very strict and condemnatory to very tolerant. Throughout Muslim history, there are more stories about acceptance from Muslim leaders and theologians than actual condemnation. In modern times, however, Muslim ideas have moved much in direction of strict and condemning.
It is not clear how Muslims shall deal with the kafirs; the Koran sura 60:8 seems to indicate that Muslims can tolerate non-Muslims as long as these do not fight against the religion of Islam.
A few passages of the Koran bring confusion into the understanding of the concept. In Koran 33:47 it is distinguished between "unbelievers" and "hypocrites" (Arabic: munāfiq (sing.), munfiqūn (pl.)). A common Islamic categorization, understands the hypocrite to be one claiming to be a Muslim, but who is lying.
Koran sura 109, called The Misbelievers, calls the faith of the misbelievers a "religion", calling both Islam and the faith of the misbelievers "dīn".
There is in modern times a debate whether a kafir and a non-Muslim is the same. By this it is alleged that a kafir were only those who had the message of Islam in the time of Muhammad presented to them, but who rejected this. With this, the category of non-Muslims is not mentioned in the Koran, and the hard criticism then does not apply to them.
Koran sura 3: Imran's house