Email marketing.
Marketing automation.
Content management and curation.
Web development.
Web design.
Copywriting and editing.
Analytics.
Business/marketing strategy.
Hey Jenny, i think your question is somewhat confusing... each part in digital marketing has its own importance.SEO,content ,keyword and all single features makes our site up.
If you want to start a career in Digital marketing then you should have knowledge in all fields.
Having knowledge in particular one field will not help you to achieve great success.
For a start, you can opt SEO and SMO. It will be a good option,