There are a number of privileges observed in law that shield evidence from discovery by adverse parties. Two of the most important privileges are the attorney work-product doctrine and attorney-client privilege.
The work-product doctrine protects documents, communications, research, and other work done by or for an attorney, whether oral or written, from discovery by the opposing party in a legal matter. Generally, the work product must have been prepared in anticipation of litigation. There is a rebuttable presumption that work product is privileged, which may be overcome if certain elements are met.
Attorney-client privilege refers to all confidential communications between an attorney and client, sometimes even when the client is only prospective. The attorney is required to observe the privilege barring exceptional circumstances, and the information is permanently protected unless the privilege is either waived by the client or through a mistake by the attorney.
The attorney-client privilege is controlled by and is for the benefit of the client, who must consent to disclosure, while the work-product doctrine is "owned" by the attorney. The attorney-client privilege is more robust than the work-product doctrine in that it is more difficult to overcome and applies to all confidential communications between attorney and client. On the other hand, the work-product doctrine can be expanded to include work done by legal assistants and litigation consultants such as forensic accountants, but the work must also be shown to have been prepared in anticipation of litigation, and the privilege is rebuttable.