The attorney client privilege is defined under California Evidence Code §954. It states that ” the client, whether or not a party, has a privilege to refuse to disclose, and to prevent another from disclosing, a confidential communication between client and lawyer.”
Put simply, this means that any communication you have had with your attorney may not be used as evidence in court. You have the right to refuse to allow any such communication to be presented by any party, and you will not be required to disclose any such evidence as part of a trial.
The privilege is held by the client and accordingly can be claimed by the client alone, however the attorney is obligated to claim the privilege on behalf of the client if the opportunity arises, unless told by the client not to do so. Furthermore, the privilege can only be claimed while the holder of the privilege is still in existence, i.e.it is only effective while the client is alive.