To fully understand copywriting, it's important to understand what it is:
1) Copywriting is salesmanship in print. In other words, copywriting should accomplish the same purpose as a human salesperson whose income is dependent on making the sale. This dictates the information that needs to be presented, the form that information will take, and the length of the message that is necessary to accomplish the purpose.
It is also important to understand what it is not:
2) Copywriting is not advertising. That is, the two terms are not interchangeable. Good copywriting is usually an integral part of good advertising, and bad copywriting is often to blame for bad advertising. But, far too much advertising as we know it is almost copywriting-free. Brand awareness and punchy image clouds or eliminates the sales message in most visual advertising, to its detriment.
In addition, three points about copywriting have proven themselves true over the last century-plus since modern copywriting began:
3) Long copy outsells short copy. More often than not, as long as the media allows it (i.e. direct mail, advertorial, infomercial) the more opportunity the copywriter has to lay out his argument, provide adequate evidence to support it, and fully explain the offer and the warranty, the better the chance it will result in a sale.
4) Benefits outsell features. The famous adage is "Don't sell the steak, sell the sizzle." In other words, people don't want to know that a new computer has 1.4 Gigantobytes of storage, they want to know they can organize all their music, photos, videos and documents with room to spare. They don't want to know that sports car they're looking at gets 32 mpg and comes with leather seats, they want to know that it will make them sexier and richer than they are now.
5) Copywriting does not have to be grammatically perfect. Getting the sale has far less to do with Strunk and White's Elements of Style, and far more to do with your audience relating to what you have to say. Without exception, a down-to-earth, conversational delivery, with common contractions (he's, you're), slang, jargon and humor will accomplish this far more than an ad that reads like a college English essay.
There are, of course, a thousand things you should know about copywriting, especially if you plan to earn any money marketing a product or service in 2009. But, that's for a different hub.