What is Riba? The Prohibition of Interest and Usury in Islam by Christine Benlafquih Jan 23, 2009

Islam provides guidelines for all aspects of life, including trade and economics. Of great importance to Muslims is the prohibition against interest and usury.

Muslims refer to the interest or usury applied to loans, credit purchases or other kinds of deferred payments as riba. Riba is regarded as unethical and unfair to the borrower, and is clearly forbidden in Islam.
Definition of Riba

The Arabic word riba linguistically means “an addition to, or an increase of, a thing over and above its original size or amount.” In the Qur’an, the term riba signifies an unlawful and forced addition to the payback value of money or goods lent from one person to another.

http://islamic-law.suite101.com/article.cfm/what_is_riba