A promissory note is a promise by the borrower to pay back the money at the agreed rate of interest by a specified date. Once it is signed, it is a legally binding contract. Some jurisdictions require a promissory note to be executed in front of a notary.
Polonius said 'Neither a borrower nor a lender be. For oft loan loses both itself and friend.' Good advice! However, if you do plan to lend money to anyone (including family and friends), then put it all in writing and signed before you hand over a penny. Otherwise you run a very significant risk of never seeing your money again.