It's where you loan out money at very high interest rates to people who are short on money and have nowhere else to turn for a loan.
It's a predatory business that preys on those who have the greatest financial needs. By charging such high interest rates, it creates a cycle whereby people can't stop taking the loans and fall more and more in debt.
I think they should be illegal in Canada. I also think there is a special place reserved in hell for anyone who owns or runs one of these - and benefits off the misery of others.