Content marketing is what you call a written piece of content that you share.
I would not call a 500 word blog post or a few photographs content marketing. If you think of content marketing like the Sunday Feature in the newspaper, the feature story where you have created a piece of content that is 2500-3000 words, has 10 or more original images, and talks about detailed sub-topics within the topic. This featured piece of content needs to invoke a response from the reader – hate, disgust, love – you need to get a response from them. Without a response you have not achieved the goal – that goal being a response. Your piece of content should let the reader come away with value, it made them laugh, taught them how to do something, provided knowledge and ideally created the response.
Now that you have created this masterpiece of content, you need to get it out in the public so people can read it. This could be sharing your content link on Twitter, Facebook, your blog, on other blogs, through press releases. This is where the marketing comes into play.
Create content, link to it, get other to publish it and when you marry the two you have content marketing.
A question originally asked on Quora: http://www.quora.com/What-is-meant-by-content-marketing