Online marketing is a term that means different things to different people so it can includee social media, online advertising, interactive promotions,  guerrilla marketing, analytics, SEO, SEM, content, etc.  There are so many tools in the marketing toolkit  today but I think it is redundant to say digital marketing because truly everything has a digital element since everyone is accessing and interacting with your brand online, through their phone or via the website at some point.   In the old days there was print, TV, radio, direct mail and outdoor those were your only options but today technology runs our lives so everything is digital eventually.  If online is not part of your strategy then you would not be relevant so digital marketing is marketing in 2020.   As  far as digital goes I am a big fan of SEO, social media especially LinkedIn and Content Marketing. 

Not my area but I'd be interested to hear from @Jef Lippiatt @Maini Homer @Steve Gray @Bronwen Sciortino @Theodora Nikolaou @Calum Maxwell @Beau Ushay @Paige Arnof-Fenn @Tim Stokes