Home loans and student loans are two examples of promissory notes.
Promissory notes: A promissory note is a written agreement to pay back a debt in a certain number of payments, at a certain interest rate, and by a certain date and time. For instance, medical bills may be considered a written contract, even if no repayment terms are included and the bill itself doesn't contain any signatures. Often a written contract includes the terms and conditions of the loan, such as the loan amount and monthly payment.
Written contracts: All debts that come with a contract that was signed by you and the creditor fall in the category of a written contract-even if it was written on a napkin. 
Oral agreements: These are debts that were made based on a verbal agreement about repayment, and there is nothing in writing.