I find these words very offensive and discriminatory, as they do not just refer to a person as a non-believer below I have given the most common definitions for the words kafir,kaffir,kufr, kuffar, and infidel and the sources of the definitions.

Oxford Dictionaries

Kafir -noun . A person who is not a Muslim (used chiefly by Muslims): according to the Koran, the kafir is characterized by his worldliness

Origin - From Arabic kāfir 'infidel, unbeliever'. Compare with Kaffir.

Kaffir- An insulting and contemptuous term for a black African.

Origin - From Arabic kāfir 'infidel', from kafara 'not believe'

Infidel - noun- A person who has no religion or whose religion is not that of the majority: a crusade against infidels and heretics

Origin - Late 15th century: from French infidèle or Latin infidelis, from in- 'not' + fidelis 'faithful' (from fides 'faith', related to fidere 'to trust'). The word originally denoted a person of a religion other than one's own, specifically a Muslim (to a Christian), a Christian (to a Muslim), or a Gentile (to a Jew).

Islamic Dictionary -

http://islamic-dictionary.tumblr.com/post/10098203002/kufr-arabic-is-the-term-which-describes

Kufr (Arabic: كفر) is the term which describes disbelief or denial [particularly in Islam], sometimes sometimes translated as infidelity. The term refers to a person who rejects Allah or who hides, denies, or “covers” the truth.

Urban Dictionary

http://www.urbandictionary.com/define.php?term=Kuffar

Also spelled 'kafir' or 'kaffir', Kuffar is a highly derogatory Arabic term used to refer to non-Muslims, though it is usually directed less against "People of the Book" (Christians and Jews) and more against others (Hindus, Buddhists, Shintoists, etc). 

As you will notice all the words mentioned kafir,kaffir,kufr, kuffar, and infidel are demeaning and only have a negative effect in describing someone.

That is why under current discrimination legislation, The Equality Act 2010, it should protect all UK citizens from being discriminated against by allowing people to describe others as either a kafir, kaffir, kufr, kuffar and infidel.