Callum, it's complex so if you're new to this whole flying thing, it's logical if you don't understand things.
It used to be relatively simple. There were essentially three licenses:
PPL or Private Pilots License - if you want to fly privately
CPL or Commercial Pilots License - if you want make money from your flying, but do not want to fly a scheduled airliner
ATPL or Air Transport Pilots License - if you want to fly scheduled airliners
That's the basic license. On top of this you need something called a "class" or "type" rating, that allows you to exercise the privileges of your license in a type or class of aircraft. Examples:
SEP: Single Engine Piston
MEP: Multi Engine Piston
B747: Boeing 747
In theory you could mix and match these class/type ratings with any license, so you could fly a B747 on a PPL, or hold a CPL with just a SEP class rating.
Then you have additional ratings. The most important one is the Instrument Rating, which allows you to fly the aircraft with sole reference to instruments. Night is also a Rating these days, which allows you to fly outside daylight hours. The Night Rating used to be a prerequisite for the IR, but with the most recent EASA proposals that's no longer the case. There's also an Aerobatics Rating right now, and a few others.
The last component of your license is your medical. PPL requires a medical class II, anything higher requires a medical class I. Medicals differ in the number of tests, and the norms applied to each test, but also the frequency with which you need to be tested.
So a full pilots license will consist of the following:
- License (PPL, CPL or ATPL)
- One or more type or class ratings
- Additional ratings such as IR, Night, Aerobatics
- A medical.
Only once you have all these ducks in a row for the specific aircraft you're going to fly, and flight conditions you're going to fly in, are you allowed to take off.
So far so good.
Now the regulators have come up with all sorts of oddities and exceptions to this. At the low end, there's the NPPL, which is a sub-PPL UK-only rating. It doesn't require an aviation medical, just a GP declaration, and it limits you to day VFR and a limited set of aircraft. The NPPL will soon be replaced by the EASA LAPL. Together with the NPPL the CAA also brought in the term "Microlights", which is a subcategory of the SEP class rating.
Glider pilots have also been brought into the fold by EASA. But a Glider Pilots License is not simply a PPL (or NPPL) with a "Glider" class rating. Lots of differences. Then there's the crossover types like SLMGs and TMGs which, within limits, can be flown on both a GPL and a PPL, or where holding a GPL or PPL will get you considerable credit. Oh, and this reminds me of more crossover stuff: If you hold both a SEP and a TMG class rating, revalidating one also revalidates the other.
At the high end, you cannot simply get an ATPL with just a SEP class rating. An ATPL implies that you have an IR, have done the Multi-Crew Coordination (MCC) course, and have at least one multi-engine type rating. A full ATPL can also not be issued until you have a certain number of hours.
Still following? What it still comes down to is that you have to have a license (NPPL, PPL, CPL, ATPL, GPL, whatever), plus the appropriate class or type ratings for the aircraft (SEP, MEP, B747, whatever), plus you have to have additional ratings like the IR, plus you have to have a medical appropriate for whatever you're doing. But all these bits of paper may be dependent on each other in complicated ways, and certain bits of paper need other bits of paper, for them to be valid.
The UK used to bring out a publication called "LASORS" which is almost 1000 pages, and deals almost exclusively with flight crew licensing. Now that EASA has taken over flight crew licensing it's no longer valid and no longer kept up to date, but most of the stuff in there is actually still describing the current situation pretty well. After all, both CAA/JAR-FCL and EASA-FCL are based on the principles set by ICAO. It should still be available on the internet, and otherwise I'll send you a copy.
So, yes, it's complicated and subject of endless debates on here. But once you start learning to fly, your instructor will guide you through obtaining the necessary bits of paper as you go along, meaning that when you do pass that final test, you are indeed fully legal to fly. (That is, after the CAA has processed the paperwork for you.)