SEO can be broken down into 2 main categories, on-page optimization, and off-page optimization. On-page optimization involves the changes that are made on your actual site, like through a CMS such as MODx. Changing heading tag, title tags, alt tags, etc., are all examples of on-page optimization. Off-page involves anything and everything that isn't actually done on your site, such as social media, blogs, forums, etc.