Content marketing is a broad strategy that targets users across various channels (search, social, email etc) and engages users as they move between them. Without these channels, nobody would be able to see your content and a key part of your strategy is understanding which channel(s) to deliver each piece of content on and how to target users on them. SEO is about making your content as accessible as possible for search engine users and helping it perform effectively. This starts with optimizing your pages and content for visibility in the searches that matter most but also involves broader, technical factors that affect how people engage with your pages (mobile optimization, loading times, etc).