A contractor is a human which made blood contract with a demon. By doing that the human gains enhanced physical powers and is able to use a bit of the power the demon. The demon on the other hand can use his full power in human world, which is normaly not possible. A contractor seems to be always in range of this demon to use his power.
If the contract was a success, is showing when by the human a seal with the symbol of the demon appears. This seal can get bigger when the human use more of the demons power. By doing this the human itself looks more and more like a demon. This technique is also called Black Techs.