It's very dangerous, you should stay away from it. Look what it has done to the other posters on this thread. they don't seem very content, do they?
Article writing is when someone comes behind you offering to write $1-$3 articles. Copywriting is when you go behind a writer and offer $$ for an article.. BTW I am looking for copywriters
Professional copywriting involves writing text that others cannot understand, yet appreciate. @Websolveit: Kudos
Get a copy of E.B. White's "The Elements of Style." It remains the quintessential guide for writing well in English. Should be available online or at any library, for free. Ask the librarian. To answer your question (in hopes of making a dent in the widespread misinterpretation of this concept): Copywriting is first and foremost a function of writing. Then, ideally, the purpose of sales and marketing is applied, rather than the other way around.