In general there are two types of debts that specialists distinct from each other: secured and unsecured.
Unsecured debts are:

• medical bills
• credit cards
• department store cards
• personal loans
• student loans
• bounced checks

Secured debts are:

• home mortgage
• auto loans

When speaking about a secured debt, your real property (car or home) is used as a collateral for the loan and is collected if you default on it. Such debts cannot be settled because the creditor will simply repossess your property in case you get behind with your payments substantially.
But when speaking about unsecured debts, there is no collateral whatsoever that can be used to secure the payment. Such debts are usually given to people with a good credit history simply for trusting their creditworthiness. The creditor will want to settle such debts, because there are no other guaranties from your part that you will settle the payments (outside legal actions, of course). So you can effectively use debt settlement to work out this type of debt.