A non-disclosure agreement is a legal contract between several parties that establishes the confidentiality of shared knowledge or materials and restricts third party access. In lay terms, business associates use non-disclosure agreements to make sure neither party is allowed to speak about or divulge covered aspects of the agreement to anyone else.
Non-disclosure agreements are generally used when two or more businesses or individuals chose to conduct some sort of business together or must share confidential information. In these cases it's important to make sure all privilege information is only shared with the appropriate people.
Need access to a free non-disclosure agreement? A blank sample is available from Harvard Business School here in the form of a pdf file.