Yes, of course, I'll be happy to explain the difference.
If you wish to undergo a course of training that gives you a useable qualification, you are entitled to take your trade to whoever offers such training. In the airline world, a type rating on its own is only half a qualification. If you want to offer yourself as qualified on an airliner type, you need the line training that effectively ratifies the type rating. In most shorthaul airlines, line training is around 40-50 sectors, or roughly 100 hours. At the end of that time you are awarded an 'area and airfield' ratification, which allows you to operate to anywhere in that airline's network. At that point, you should be considered to be a competent, though probationary, operator of that type. That is the limit of training in or for an airline, and should be the limit of training offered or contracted for commercially. This line training is always conducted on revenue sectors; the costs otherwise would be prohibitive for airline and trainee. Whether such a course is a worthwhile investment is for you to judge, but the existence of such courses is more or less justifiable - though a sad comment on the way things have changed over the last 30 years.
Any 'line training' or 'co-pilot' schemes that offer more than this 100-hour period are effectively using students to subsidise their costs, as there is no official line training course that requires any more than the hours I've mentioned. 300 or 500 hour 'courses' are simply exploiting your insecurities to get a free (or, rather, paying) first officer. That is immoral, and should be prohibited.
Please do not get the idea that I am defending the system that makes self-funded type and line training tempting for ab-initio pilots; I would far rather airlines paid these costs. However, I entirely understand the financial pressures that have brought this about. Also, for the experienced pilot (such as myself), type and line training deals can be a very cost-effective way of getting a job we are not currently qualified for. That is the nature of speculative training - you do it to make yourself more employable. It would be no different if I decided that I now wished to be a teacher or a lawyer; I would have to pay the entire training costs myself - but I would not be expected to pay to practise either once fully qualified!
Scroggs