Pilot

Commercial Pilot Licence with Instrument Rating (CPL/IR) - The traditional ab-initio method of training pilots for the airlines. The CPL/IR are the bare necessities for anyone thinking of a career in the airlines. Trainees spend about 200 hours training on small general aviation aircraft. While in Singapore we think of commercial pilots only as those who helm the large jets taking off and landing at Changi, the term 'commercial pilot' technically means any pilot who flies for financial compensation (pay) for their services which can range from corporate jet charters to crop-dusting.
Anyway, to get to the airlines, CPL/IR holders must adapt from operating in a GA environment (single crew, piston engines) to an airline environment (multi-crew, turbine engine) by doing a Jet Orientation and Multi-Crew Coordination (JOC/MCC) and a type rating course (typically $20-50k on top of CPL/IR if not sponsored by airline). A type rating is like a special add-on to the licence allowing the holder to fly ONE specific aeroplane (eg: B737 TR allows you to fly only Boeing 737s.) To move on to another aircraft type, such as say, an A320, the pilot will have to get another type rating. After going through all this, the pilot starts off his/her airline career as a second officer (first on smaller airlines).
As far as the differences between the Singaporean (CAAS) and Australian (CASA) courses go, I think the Singaporean (CAAS) course comes with ATPL theory while the Australian (CASA) one does not, and on top of that, you need to do a conversion to use a CASA licence in Singapore.
Air-Transport Pilot Licence (ATPL) - Is the pinnacle of pilot licencing. It essentially allows a holder to be in command of a scheduled air career's aircraft (in English: airline captain). A CPL holder receives a 'frozen ATPL' upon passing all required ATPL exams (yet still essentially holds a CPL holder on paper). After logging 1500 hours, he/she 'unfreezes' this and receives all unconditional privileges of the ATPL.
The ATPL (Air Transport Pilot License) Theory course you mentioned is essentially what it says it is - a theory course covering all the relavant ATPL theory. With that being said, I believe this course is part of the CAAS CPL/IR course.
Multi-Crew Pilot Licence (MPL) - Because of the anticipated rapid growth in global air travel, ICAO devised an alternative all-in flight-training method to streamline pilot hopefuls to the airlines to meet these demands. If the CPL/IR is a cheesebur

Flying pilot is an excellent career option. There is a general shortage of trained and experienced pilots worldwide. A majority of pilots requirement is in Asia Pacific region. Commercial pilots are paid one of the best salaries in India with other perks.
Following are two options to become pilot in India
(a) Commercial Pilot: You can become a commercial pilot by joining pilot (flying) training schools. Following are the basic eligibilities criteria to become a commercial pilot:
a. You should have completed the 10+2 years of education with physics, chemistry & maths as your major subjects.
b. You must have scored 50 percentage of marks in above-mentioned education
(b) Flying Pilot in Indian Air Force.
You can join Indian Air Force flying branch by appearing for NDA after 12th board or CDS after a science graduation degree.