The title chief of staff (or head of staff) identifies the leader of a complex organization such as the armed forces, institution, or body of persons and it also may identify a principal staff officer (PSO), who is the coordinator of the supporting staff or a primary aide-de-camp to an important individual, such as a president, or a senior military officer, or leader of a large organization.
In general, a chief of staff provides a buffer between a chief executive and that executive's direct-reporting team. The chief of staff generally works behind the scenes to solve problems, mediate disputes, and deal with issues before they are brought to the chief executive. Often chiefs of staff act as a confidant and advisor to the chief executive, acting as a sounding board for ideas. Ultimately the actual duties depend on the position and the people involved.
In general, the positions listed below are not "chiefs of staff" as defined at the top of this page; they are the heads of the various forces/commands and tend to have subordinates that fulfill the "chief of staff" roles.