That's like asking, whether a cop is an enforcer or a protector? The answer is - both. A good copywriter is someone, who sells in the middle of the conversation with the customers. I've seen some superb storytelling and people clicking "buy" links everywhere throughout that story. That's some premium copy. Unfortunately, it's pretty rare. I think someone, who writes copy should actually believe the stuff that they're writing. That's why customers are the best brand advocates - they know that the product/service is great and they want to share it with other people. You can always see fake, empty copy from a mile away. I'm not talking about the quality, in terms of grammar. I'm talking about the quality in terms of sincerity and the actual knowledge of the product.
Some copywriting jobs involve selling and some do not. A good copywriter needs to be able to write to achieve the goals of the project, whatever they may be. I may want an article to educate, position a company or product, establish thought leadership, or other such functions and specifically not want to "sell" anything as a result of the copy. Of course, all copywriting, by definition, is communication.
Copywriter must be a good communicator, and must be best in persuasion--can make buyers buy or avail without really hard selling them. That's the art in it.
A copywriter writes advertising. Basically he/she is a salesman in print. Copywriting is an art, the art of writing persuasive ads that will hopefully induce people to part with their money. Different target markets require different forms of communication, so the copywriter must be adaptable and be good at pinpointing the type of research required.
Both. A true expert will also be able to tailor his product to one direction or the other, or any point in between, depending on what the user is looking for.