However the slight differences can make a difference in how well the content you receive meets the goal you had in mind.  To help you distinguish the difference here are a few basics on each of these so that you can make sure your writer will suit your needs in particular.

Copywriter- The goal of a copywriter is to convince or persuade a reader that the information on your website as relates to a product or service is absolutely true.  The intention is to educate the reader to reach the conclusion that you are an authoritative source who knows what is best for them, which incidentally would be heeding their advice or purchasing a specific product.  In other words, copywriting is heavily sales orientated.

SEO content services

Bear in mind that SEO means Search Engine Optimization and you can clearly see that the purpose of SEO content is to increase your overall website ranking or page value.  Thus, while you can order a slant to an article the overall purpose and goal of an SEO writer is to increase the value of your website, not to give you precise content.

Web Content Provider

The role of a web content provider is the simplest to understand, as the name implies its purpose.  A content provider simply seeks to deliver informative and tailored content for your website to fill up the site according to your demands.  Highly flexible at the right rate and with the proper negotiations you can get a mix of the top two services as well.