Usury is lending money for interest. Nowadays it usually means lending at a high, often extortionate rate of interest on a loan.
Usury is illegal in many parts of the world. In muslim countries charging any interest at all is strictly illegal.
Some people argue that this is against the Old Testament, especially Exodus and Deuteronomy which states that those who lend to someone in need will be blessed, but the point about usury is the high levels of interest, not the act of lending in itself.
Aristotle also condemned usury as "contraty to nature" in his book on Politics, because if increases by itself and makes money without labour.