The operating agreement (sometimes called a limited liability company agreement) is the basic agreement among the members of a limited liability company that governs the operation of the LLC. The operating agreement will set forth the relations among the members, the rights and duties of members and managers in governing the LLC, and the financial interests of the members in the LLC, among other things.
The operating agreement is really just a contract entered into by all of the members of the LLC, but has a number of unusual features not typically found in contracts. For example, the LLC itself is not normally a party to the agreement, but is bound by the agreement. Furthermore, in a single member LLC, the operating agreement can be entered into by a single person.
The formation of the limited liability company is accomplished by filing a certificate of organization (called a certificate of formation in Delaware). The certificate of orgnaization is a skeletal document that does not contain all the detail necessary for the governance of the LLC. Thus, the operating agreement is the document that fills in that detail. Unlike the certificate of organization, which is filed with the state to form the LLC, the operating agreement is not filed with any state agency, allowing the LLC's internal financial and management details to remain largely private.