In law, oaths are made by a witness to a court of law before giving testimony and usually by a newly appointed government officer to the people of a state before taking office. In both of those cases, though, an affirmation can be usually substituted. A written statement, if the author swears the statement is the truth, the whole truth, and nothing but the truth, is called an affidavit. The oath given to support an affidavit is frequently administered by a notary public who will memorialize the giving of the oath by affixing her seal to the document. Breaking an oath to tell the truth, the whole truth, and nothing but the truth is perjury.
Various religious groups have objected to the taking of oaths, most notably the Religious Society of Friends (Quakers) and the Mennonites. This is principally based on the words of Jesus in the Sermon on the Mount: "I say to you: ´Swear not at all´" (Matthew 5:34). Jesus says that we should hold ourselves to a high standard of truthfulness at all times: "yes" should mean "yes," "no" should mean "no," and there is no need for elaborate formal promises. Not all Christians follow this reading, because there are other Biblical statements which mention oaths without condemning them.
Opposition to oath-taking caused many problems for these groups throughout their history. Quakers were frequently imprisoned because of their refusal to swear loyalty oaths. Testifying in court was also difficult. George Fox famously challenged a judge who had asked him to swear, saying that he would do so once the judge could point to any Bible passage where Jesus or his apostles took oaths. (The judge could not, but this did not allow Fox to escape punishment.) Legal reforms from the eighteenth century onwards mean that everyone in the United Kingdom now has the right to make an solemn affirmation instead of an oath. The United States has permitted affirmations since it was founded; they are even mentioned in the Constitution and Franklin Pierce availed himself of the privilege to affirm rather than swear when he was inaugurated president. (He is the only president to have done so.)
Because of this new legal situation, a few Quakers now believe that there is no real difference between an oath and an affirmation, other than the word used. Consequently, they refuse even to affirm using a set phrase.
http://en.wikipedia.org/wiki/Oath
Oath c´est "serment" (enfin t´auras compris je pense)