They are usually taught by ex-military. Very often the instructors held highly regarded positions within various military organizations. Often times the instructors were formally Drill Instructors, Special Forces Operatives or have otherwise distinctive careers.
These courses always have some common elements. They often focus on military style calisthenics and group runs. The courses are often held very early in the morning and will meet in almost any weather. Students can expect pushups, situps, pullups, and jumping jacks, as well as more obscure drills such as flutter kicks, sun worhipers and flares. Almost invariably a workout will include short runs while longer runs are more scheduled.
One of the oldest courses is The Seal PT Course which is held in Houston and in New York. These courses have been growing more popular lately and are available in more and more locations across the country. SealFit is a Military Fitness website that maintains a list of courses in various areas, as well as facilitating communication between the students of various programs.