A promissory note is a written unconditional promise by the maker to pay a certain sum of money to the payee or bearer. It must be signed by the maker and include certainty of parties and amount of money. The maker is the person who promises to pay, the payee is the person the note is payable to, and the holder is anyone entitled to possession and payment. Key characteristics include being in writing, containing an express promise to pay, and being definite, unconditional, and for a certain sum of money only. Formalities are not essential for a valid promissory note.