...oh boy
Well the first thing you need to understand is the difference between a licence and a rating.
Licenses:
NPPL
PPL
CPL
ATPL
Ratings:
SEP
Night
MEP
Twin
IR
Type Ratings eg (Dash 8, B373 etc)
Others include: tailwheel, seaplane
To fly you need a licence + a rating ie when you initially fly privately you have a PPL + SEP.
To get a job in the RHS you generally need what is called a frozen ATPL which consists (from zero hours);
Modular: PPL + Night + ATPL ground exams + CPL + Twin + IR + MCC (Multi-crew course)
Integrated: All of the above training thrown in together to get an f/ATPL
There are various hours requirements at each stage as well as a 300 nm cross country flight to get issued with your CPL.
Modular schools will sell you the individual bits, Integrated schools the whole shabang.
There is also the MPL but that is only airline sponsored, I believe