Start your reading here: http://en.wikipedia.org/wiki/Copywriting There are also links to different types of writing.
Not to sound trite, but the difference is several hundred (or thousand) dollars per page. Copywriting requires a level of expertise that content creation does not (no offense to article writers here). To be able to write good copy, you also have to be able to sell. You have to have a certain grasp on consumer psychology in order to write to a very specific audience with each new project. Content creation simply doesn't require you to do that. Think of it this way: if you were to need brain surgery, you would want a neurosurgeon to perform the operation, and not your general practitioner. Sure, both are doctors, but one has a level of expertise that the other does not. Copywriters and content creators are both writers, yes; but copywriting is a much more specialised practise than the other. Hope that helps