The difference is that Content Writing Is More General, and Copywriting Is Specialized, is mainly used for seo. In other words, copywriting is content writing taken to another level.
copywriting involves strategically writing words that promote a person, product, business, opinion, or idea, with the ultimate intention of having the reader take some form of action.
for content writing I suggest you read the article here..
Content writing, I believe, is (literally) writing a content. To write whatever you want and feel. In Copywriting, especially in SEO copywriting, it is the strategical placing of keywords/keyphrases inside your content and still be be able to generate/create a readable and quality content.
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"Copywriting is the use of words to promote a person, business, opinion or idea. Although the word copy may be applied to any content intended for printing (as in the body of a newspaper article or book), ..." http://en.wikipedia.org/wiki/Copywriting