Riba is the (Arabic: ربا ) term for usury, the charging of which is forbidden by the Qur'an. For example, Sura Ar-Rum (30:39) states:
And that which you give in gift (loan) (to others), in order that it may increase (your wealth by expecting to get a better one in return) from other people’s property, has no increase with Allâh; but that which you give in Zakât (sadaqa - charity etc.) seeking Allâh’s Countenance, then those, they shall have manifold increase.
And Sura An-Nisa(4:161):
That they took riba (usury), though they were forbidden and that they devoured men’s substance wrongfully – We have prepared for those among men who reject faith a grievous punishment.