One of the most common questions and misunderstandings I come across with new Web writers is about Web content versus Web copy. If you want to write for the Web, it's important to figure out what type of writer you want to be. Understanding the difference between content writing and copywriting will get you off to a good start.
I'm going to over-simplify for the sake of clarity here, but there can occasionally be pieces that cross the lines. However, in most cases here is what content is / does, and the same for copy:
Content - Content is designed to inform, educate, or entertain in most cases. For example, a how-to article is content.
Copy - Copy, on the other hand, is generally designed to persuade or entice the reader to action. This can be blatant such as marketing copy telling people to buy your product or sign up for your newsletter or click on some link. It can also be much more subtle. For example, the homepage copy on a business website is copy rather than content, as would be the text on something like an "About Us" page. Why? Because that copy is designed to give a certain image of the company - you're persuading them to trust you by giving background information that helps them make a decision put before them by your more overt copy. The overt sales-oriented copy might be referred to as direct response marketing copy, but not being direct response copy doesn't make the rest any less copywriting.
To be a great content writer in a niche, your best assets are your knowledge of the subject matter, your credentials that make you a trusted source, and your research ability. On the other hand, to be a great copywriter, you not only have to be able to write, but you have to be able to persuade - you should have a solid grasp of basic marketing and consumer psychology.