What is protected under attorney-client privilege?

Attorney-client privilege protects certain communications between a client and their attorney. According to the Legal Dictionary, attorney-client privilege gives the client the privilege to refuse to disclose information that the client conveyed to the their attorney.  The same privilege to refuse disclosure is afforded to the attorney if they are asked what the client told them during a confidential discussion.
Attorney-client privilege gives the client full ownership over the information they share with their attorney and only the client has the right to waive it. 
Keep in mind, there are many exceptions to attorney-client privilege. For example, if you tell your lawyer that you're going to physically or economically harm another party, then that communication is not protected. Another example is the common interest exception. This is when the same attorney, in a case, represents two parties. In that situation, neither client may assert the attorney-client privilege.