Digital marketing is a vast market with many sub markets and domains. But digital marketing is mainly any advertisement used online, video, radio and tv.
While SEO is not marketing it is so your site can be optimized to rank higher in search engines such as google, bing and yahoo among others.
There is also SEM(search engine marketing) another broad topic of advertisement and sub domain of digital marketing. SEM is basicly marketing through keywords of a search engine and other ranked website’s of those keywords.