Digital Marketing and SEO (Search Engine Optimization) both tech/art use for online marketing. some people consider synonymous both term but both are different and good companies hire different candidate for both profiles.
SEO: Search engine optimization technique/art use to optimized website ranking on SERP (Search Engine Result Page) for specific Keywords/Search Term. There are some things which people do for SEO Like: On page SEO, OFF Page SEO, Technical SEO (Website Technology base optimization). There are no limitation in SEO.
Digital Marking: Digital Marking is another technique on online/web base marketing but it is different from SEO. As We have to pay for Digital Marking not whom, who is doing digital marking, but we have to pay whom, which services we are using Like: Google, Facebook etc. As we run paid ads on Google, Facebook that is Digital Marketing.
In Simple words SEO is Free and Digital Marketing is Paid.