The term jet charter is a catch all phrase that refers to the renting of an entire aircraft vs. individual aircraft seats. While the Airlines specialize in selling transportation by the seat, jet charter companies focus on small groups for specialized itineraries, urgent / time sensitive freight or cargo, air ambulance and any other form of ad hoc air transportation.
Generally speaking, jet charter is also known as air taxi, executive charter, air charter and more broadly as private aviation. In the United States jet charter / air taxi is governed by Part 135 of the FARs (the Federal Aviation Regulations), unlike the larger scheduled airlines, which are governed by more stringent standards of Part 121 of the FARs, which are regulations designed for scheduled air carriers.
It is believed that over 70% of the private jet charter in the world is believed to exist within the United States, the largest market for private aviation globally.