A Non-Disclosure Agreement (also referred to as a confidentiality agreement) is a binding contract that establishes a confidential relationship whereby signatories agree that sensitive information they may obtain will not be made available to any third party. They should be used prior to negotiations to allow parties to share sensitive information without fear that it will end of in the hands of competitors or others who will abuse it.
Note: NDAs are not a protective panacea. It is impossible to “un-ring the bell” once you disclose sensitive information, whether there is an NDA in place or not. You should only ever provide information to third parties that they need to know to carry out or evaluate the proposed transaction.