Hiya gingernut,
Their are lots of careers in flying: Corporate, cargo, airline, airborne medical, police, flying instructor.... all require the pilot to hold a CPL (see below).
i am no expert, if you go to cabairs website or any other flying schools website they do a pretty good job of explaining the path. I will give it a go but someone may wish to correct me, I am not sure on some things like the limit weights:
PPL: Allows operation of single piston engined aircraft under (5200KG BOW???) in VFR. (You know this already as you hold a PPL?)
Night rating: Allows flight with passengers after sunset and before sunrise (ie at night)
IMC: extends the rights of a PPL to allow you to fly in IMC
IR: Instrument rating, allow flight in IFR. Airlines require the applicant to hold an IR in 99.9% of the case, an IR in terms of 'pecking order' negates the need to gain a night rating or IMC rating because it is inclusive of the training for these ratings.
Multi Rating: Allows the holder to fly multi engine piston aircraft upto (5800 KG ???)
CPL: Commercial Pilots Licence, allows the holder to be rewarded for his flying. The holder may be pilot in command of SINGLE pilot operational aircraft under the privilages of ratings held. (for example if you did not hold an IR you could only be paid to fly in VFR conditions) This is why airlines require at least CPL with IR/Multi.
fATPL: Frozen Airline Transport Licence, The holder may act as first officer on MULTI pilot opperational aircraft. Licence is issued on the completion of ground examinations
full ATPL: Once 1500 hours are gained the holder may act as pilot in command of MULTI pilot opperational aircraft.
For any aircraft above (1.3 Tonnes???) a specific type rating must be held on the aircraft in order to fly it (ie business jets and most airliners)
Airlines also require a Multi crew co-operation course and crew resource managment course. (usually taken alongside the CPL)
A class one medical must also be obtained.
As for how to gain these licences you need to take a look at your financial situation, look at the costs of the licences your going to need and work out a plan on how to achieve them all. Of course, getting the licences is just the start, you need to build up your hours, get experience....
Good luck!
Richard