Further Reading
. The Private Helicopter Pilot's Guide
by Steve Sparrow
.  
The Art & Science of Flying Helicopters by Shawn Coyle (and the sequel)

.
Chickenhawk by Robert Mason

Qualifications

These are sometimes dictated by the insurance company, customers (like oil companies with their own safety departments), or Personnel, who mostly don't have a clue.
There are generally two grades of licence that entitle you to earn money as a pilot, the Commercial Pilot's Licence (CPL) and the Airline Transport Pilot's Licence (ATPL).
The CPL covers you for command of light helicopters, say, up to 5700 kg. You can't have one until a certain age, but you can take the exams beforehand—sometimes up to a year ahead. Your flight training will include the usual mix of night, instrument, dual, cross-country, etc., and ground training will cover Principles of Flight, Air Law, Meteorology and some technical stuff, and you may get credit for previous experience, such as aeroplane licences, military wings, and the like (because regulations change, check the details elsewhere).
You get an ATPL when you are much older and get a lot more hours under your belt. It covers you for command on anything, subject to type rating. In some countries, you can just get the hours and take the exams (USA). In others, you also need a check ride in a helicopter that requires a co-pilot (Canada). In those covered by JARs, you have to do 500 hours in a multi-crew environment, so an ATPL is unobtainable outside a company environment.
JAR
The exams take several days, and are only done at certain times of the year (roughly every two months - the results are timed so that you just fail to qualify for resits at the next round). Academically, the JAR CPL is way above the FAA and Canadian (and possibly Australian) ATPL.
JARs are changing even as they are being implemented, but, generally speaking, there are two types of study course available, integrated or modular. You need a minimum knowledge of maths, physics and English, which the school has to ensure you have, and they have to certify that you have done the ground school. You cannot mix training from different JAR states, but you can train in them as long as the State issuing your licence has approved the school.

Any variations from requirements listed below (you may have non-ICAO licences) are dealt with by the head of the training organisation, but such decisions are always subject to CAA approval.
Integrated Courses
Full time ground and flight training run by an approved organisation.
The CPL(H) integrated course does not include the IR or aerial work training. The ATPL(H) course includes multi-crew and the IR.
For the CPL integrated course, you need 135 hours of flying training, plus 550 hours of ground school, consisting of 100 hours dual with 10 cross country, 35 PIC with 10 cross-country, with a flight over 100nm, stopping at 2 different aerodromes. Also, 5 hours night (3 dual, with 1 cross-country), with 5 solo takeoffs and landings, and 10 on instruments, 5 of which can be done in a simulator. A PPL holder may be credited with up to 50% of any hours flown before the course (up to 40, or 50 if you have a night rating).