What is an officer?
In the context of a corporation, the officers generally handle the day-to-day management and control of the corporation. They are appointed by the board of directors at one of the directors meetings (either an annual directors meeting or a special directors meeting). While officers can also be directors, they generally are not. As for the specific officers a corporation has, there is usually, at a minimum, a CEO/president, a secretary and a CFO/treasurer. There can also a COO, vice-presidents, etc. While most state laws generally allow one person to hold more than one position, most corporations usually have different people hold each distinct position.
As mentioned, the officers handle the daily corporate control. This includes things such as hiring and firing employees, dealing with corporate contracts, handling bills and collections, dealing with sales and purchases, keeping and maintaining daily corporate records, etc.