Content marketing and on-site SEO are very similar, although not quite the same. Content marketing is the creation and promotion of content across the web, including things like landing pages, blog posts, articles, product descriptions, long-form content, videos, and images. This includes content on your website but also content on other marketing channels, like social media. It covers the user experience but doesn’t go as deeply into the SEO side of things.
On-page SEO is focused on optimizing the content on your website. Although creating content can be a part of it, its main focus is on ensuring the content performs well in search engines like Google, Bing, Yahoo, DuckDuckGo, as well as other search engines like YouTube and the App Store (depending on what the content/SEO is focused on).