If you fail to meet the repayment terms of your contract with a creditor, you will receive a number of letters informing you of this failure and requesting that the terms be met and payments be made. If the creditor fails to get you to conform to the terms, they will very likely send your case to a collection agency.
Collection agencies work on behalf of the creditor to recuperate the amounts owed by debtors.
It is important to understand that the agency's interests lie with your creditor and not with you. It is also important to understand that these agencies are highly professional and experienced in dealing with the psychology of debtors. Collection agencies get a percentage of debt collected. They may charge other fees for time they put in, so may tend to be aggressive. A few may go overboard, which can work in your interest.