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.