In the United States, USPA issues four skydiving licence classes — A, B, C and D — in ascending order. Each requires a minimum number of jumps plus demonstrated skills, and each opens up privileges the previous one did not.
Roughly what each one means
The A licence is the first, marking the point at which someone can jump without direct supervision and typically requires around 25 jumps along with specified freefall and canopy skills.
B, C and D follow at increasing jump numbers, adding requirements such as night jumps and water training, and acting as prerequisites for instructional ratings and for some competition and demonstration jumping.
Exact requirements change as USPA updates them, so treat this as orientation and check the current Skydiver's Information Manual for the specifics.
Licences travel, with paperwork
Most countries' systems recognise equivalent licences through the international federation, but drop zones will still want to see your licence, logbook and often a reserve repack in date before letting you jump.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.