Corporate officers are the individuals appointed by the board of directors with legal responsibilities within a company. Most often, companies have at least the two officer positions; President and Secretary. Some companies may also have a Treasurer.
The company bylaws or a resolution of the Board of Directions lays out the specific responsibilities of these officers and their titles. They are almost always able to sign documents on behalf of the company, but the bylaws and charter will specify this.
It's a popular misconception that the President is actually in charge of the company. However, the President is not always the person who's most important in a company. The president and CEO may be the same person, or they may be different people. Just as the CFO and Treasurer could be the same or different.
For many young startups, especially ones started by a lone founder, all three positions of President, Secretary, and Treasurer are held by one individual.