What is "Attorney-Client Privilege?"

The American Bar Association has some helpful information defining the rules and scope of attorney-client privilege. It also includes the relevant court cases for further / deeper study.

What is the privilege?

Under principles of federal common law, the attorney-client privilege protects confidential communications by a client to an attorney made in order to obtain legal assistance from the attorney in his or her capacity as a legal adviser and the advice given by the lawyer in the course of representing the client. SeeUnited States v. Amerada Hess Corp., 619 F.2d 980, 986 (3d Cir. 1980); In re Benun, 339 Bankr. 115, 126 (Bankr. D.N.J. 2006); In re Universal Service Fund Telephone Billing Practices Litigation, 2005 WL 3725615 at *5 (D. Kan. July 26, 2005):

What are its essential elements?

Under federal common law, the essential elements of the attorney-client privilege are: (1) where legal advice of any kind is sought (2) from a professional legal advisor in his capacity as such, (3) the communications relating to that purpose, (4) made in confidence (5) by the client, (6) are at his instance permanently protected (7) from disclosure by himself or by the legal advisor, (8) except if the protection be waived.

It's important to note that not every aspect of the case is protected by the privilege.

Only the communications pertaining to advice, and not the underlying facts, are protected by the privilege. Upjohn Co. v. United States, 449 U.S. 383, 389 (1981); Williams v. Sprint/United Management Co., 2006 WL 1867478 at *5 (D. Kan. July 1, 2006) ("The protection of the privilege extends only to communications and not to facts. A fact is one thing and a communication concerning that fact is an entirely different thing."); United States v. Keystone Sanitation Co., 885 F. Supp. 672, 675 (M.D. Pa. 1994) ("attorney billing statements and time records are protected by the attorney-client privilege only to the extent that they reveal litigation strategy and/or the nature of services performed."); Philadelphia v. Westinghouse Elec. Corp., 205 F. Supp. 830, 831 (E.D. Pa. 1962). As the Supreme Court explained in Upjohn: "The client cannot be compelled to answer the questions, ‘What did you say or write to the attorney?', but may not refuse to disclose any relevant fact within his knowledge merely because he incorporated a statement of such fact into his communication with his attorney." 449 U.S. at 396.

Privilege is a response to a request by the opposing counsel. It's similar to--but not the same as-- confidentiality, which is an ethical obligation on the attorney to keep interactions with her client "close to the vest."

The ABA's definition of confidentiality can be found here.