A Non-Disclosure Agreement (NDA), also referred to as a Confidentiality Agreement is legally enforceable agreement in which parties agree on the type of information, documents or details that are to be considered confidential and secret. The parties then agree not to disclose these information, documents or details to others.
An NDA could be mutual (i.e. both parties disclosing and confidentiality obligation binding on both parties) or one-sided, (i.e. only one party disclosing and confidentiality obligation will be binding on only the recipient of the confidential information).
Typically an NDA will create some exclusions on some information (e.g information which becomes public through no fault of the receiving party) and some people (e.g the legal or financial advisers of the receiving party, or information requested by a regulatory body)