An Operating Agreement is an agreement between members of a Limited Liability Company (LLC) akin to Corporate By-laws in a Corporation (but with differences). No state requires an LLC to adopt an Operating Agreement, and there are no rules of what needs to be included in an Operating Agreement if one is adopted.
Really, an Operating Agreement is just a contract between the members, and as such can include pretty much anything, but often includes details about member’s financial and management rights and duties. Note that States have default rules for such rights and duties that are contained in the state statutes and case law. As such, in situations where no Operating Agreement is adopted or in situations where an adopted Operating Agreement does not supersede these default rules, the default rules will govern.