Articles of Organization and Operating Agreement
Articles of Organization is an official document filed with the state which in essence creates the LLC under the laws of that state. Articles must be drafted to meet the State requirements to form an LLC. If you have the paperwork you have received from the state or from your Organizer you should see one that states "Articles of Organization" (in some states it would be Certificate of Organization, which is essentially the same).
Operating Agreement is an internal company document, not filed with the state, that governs the affairs and many aspects of the life of the company (such as membership terms etc). Usually this document is drafted by an attorney, however there are services such as MyLLCAgreement.com that allow you to draft a customized Operating Agreement for a fracture of what it would cost with an attorney (however, it is still often recommended to consult one).
Hope that helps.