A promissory note is a specialized form of a contract, governed in part by some special rules in Article 3 of the Uniform Commercial Code but otherwise subject to the law of contract.  Its most basic feature is a promise by one party to pay a specified sum of money to another party at a specific time.  The special rules concerning promissory notes are generally considered in an upper division course entitled "Payment Systems" or "Commercial Transactions" in which you also study about checks, electronic payment systems, and letters of credit.   A footnote in the opinion in Tahoe National Bank v. Phillips gives an example of a promissory note.