This is a common question and to do so understand it successfully, it is important to understand the difference. Content marketing is a holistic approach which reaches consumers through different channels (search, media, email, etc.) and connects users as they travel within themselves.

SEO aims to make the site as open and effective as possible for search engine users. It starts by optimizing the pages and content for success in the most relevant keywords, but also includes wider, technological factors affecting how people engage with your sites (mobile optimization, loading times, etc.).