Riba is usury. Usury is commonly understood as the lending of money at illegally high rates of interest. But riba, or usury, in Islam is the lending of money on interest, regardless of the rate of interest! When money is lent on interest, then money by itself, independent of any labor or effort, or the assumption of any risk, increases over time. The increase is realised through the exploitation of labor, goods or property since Allah, the Most High, has unambiguously declared that nothing can be had without effort or labor(6:85, 11:85, 26:183, etc.).
Riba is also at work when wealth is sucked from the masses through deception and other forms of fraud. Such, for example, is the case of paper, plastic and electronic money. There is also riba in speculative transactions. More than 60% of all money transfers in the world economy today represent speculative transactions.
"O believers, fear God, and give up the interest that remains outstanding if you are believers. If you do not do so, then be sure of being at war with God and His messenger. But, if you repent, you can have your principal. Neither should you commit injustice nor should you be subjected to it". (The Quran 2:278-279).