Most charter operators run under FAR Part 135, which is titled "Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons On Board Such Aircraft."
FAR Part 135: Applies to turbojet engine powered aircraft with 1-30 seats, non-transport category turbo-propeller powered aircraft with 10-19 seats, and transport category turbo props with 20-30 seats. So as you can see it's for smaller aircraft, under 30 seats. Applicants for a FAR Part 135 certificate must have exclusive use of at least one aircraft. FAR Part 135 is more restrictive than FAR Part 91, but some single aircraft owners might consider adding an aircraft to someone else's FAR Part 135 certificate. In this case however, the concept of operational control is important. The certificate holder is responsible for operational control, and must be able to document and show operational control over the crew and aircraft. FAR Part 135 covers charters, but not brokers. Here are a few other important aspects of FAR Part 135.
Roles: FAR Part 135 defines several important roles. The certificate holder is the entity that has applied for and holds the FAR Part 135 certificate. It also specifies both a pilot-in-command, and a second-in-command (which is not always needed). The pilot-in-command must have a minimum of 1,500 hours of experience and must remain in command for the entire flight.
Aircraft requirements: The aircraft must be in an airworthy condition, including meeting the requirements relating to identification and equipment. It must carry an appropriate and current airworthiness certificate.
IFR and VFR: A major differentiation specified in FAR Part 135 is whether the pilot is using IFR (instrument flight rules) or VFR (visual flight rules). Visual rules can be used when the pilot can use his sight to fly the plane, while instrument flight rules involve the use of flight instruments to operate the aircraft rather than sight. Many of the specific rules and regulations of FAR part 135 differ depending on which method is being used.
Manual and Recordkeeping: Part 135 requires the existence of a manual, and details what must be included in the manual, including procedures for accident notifications, and reporting and recording requirements. Certificate holders are also responsible to a specific level of recordkeeping at their place of business. Required records including operating certificate, operating specifications, medical certificates, and records of each pilot.
Staffing and Training: To maintain the safety of aircraft and passengers, specifics on staffing and training are outlined in FAR Part 135. There's a specified minimum flight experience required for pilots operating under a FAR Part 135 certificate. In addition, a second-in-command is required when carrying passengers under IFR unless approved to use autopilot. For planes offering more than 19 passenger seats (excluding the pilot), a flight attendant is required.
Testing: Testing and training procedures are also detailed. All pilots must be regularly tested on all aspects of flight including (but not limited to) weight and balance of each plane, air traffic control procedures, meteorology, recognizing and avoiding bad weather, instrument proficiency under IFR. Flight attendants must also be tested. Certificate holders must have a training curriculum as described in Part 135, which requires the appropriate facilities, instructors and simulators. Part 135 also describes the specifics on what the curriculum should include.
Safety procedures: For the safety of aircraft passengers, FAR Part 135 details passenger information, including procedures on smoking, the serving of alcoholic beverages, and restrictions of who can occupy a pilot's seat. Required equipment, which varies depending on VFR and IFR, are also detailed.