An Agenda is a list of the items of business to be discussed in a meeting. Or, more generally, a list of business items to be disposed of in some way before a specified time.
There are many ways to organize an agenda, and these may be specified in the organization Bylaws. If not, Robert's Rules of Order suggests how this can be done. Or the organization may have adopted some method by custom. For example, the meeting may open with a short statement from the Chairperson, then a review of the previous meeting's minutes, followed by reports from the principal officers, reports from ad hoc officers, then old business and new business. Other agendas might be arranged around the department structure, with reports and business organized within each subsection.
The key to a good agenda is that it be inclusive of all relevant business for the meeting at hand, and that it be published in advance so that the participants can be prepared and effective at the meeting.