Many pilots will, after getting the Private Pilot license, add other ratings or licenses to their capability.
The most common rating addition is the Instrument Rating. This rating allows the pilot to fly in weather conditions that the non instrument rated pilot isn’t allowed or capable of flying in.
A Commercial license may also be obtained. With this license the pilot may charge for their piloting services and, of course, those flying for the airline or airfreight operations must hold such a license. Flight of large aircraft (more than 12,500 pounds) or pure jets require a type rating that is specific to that particular aircraft design.
As a pilot increases their experience and utilization of an aviation pilot certificate, one usually has a desire to fly aircraft that are capable of more speed and/or load carrying capacity. If one wants to fly an aircraft that that has an engine exceeding 200 horsepower a 'high performance' rating is required. This is simply an logbook endorsement after required training (often determined by insurance companies) is completed that allows PIC (pilot in command) operation of that aircraft.
In order to gain the utilization of an aircraft capable of higher speeds, useful for long cross country travel, one must gain a 'complex' endorsement in his/her logbook. These aircraft would have retractable landing gear and a constant speed propeller (one that changes in pitch) and may or may not also be 'high performance'.