A
loan shark is a person who lends money to somebody (usually a very
poor, very
desperate person) with the promise of receiving the
original sum and X amount of
interest in return. X can be anything from 1
% to 1000
% depending on the
shark.
Violence and intimidation would be the main weapons of a loan shark in order for the money to be returned, and when that fails, the violence usually spreads to the person's closest family and friends.
It's a very nasty business and has been going on for years in the poorest parts of the world.