I dont think there is any diff between online marketing and digital marketing. If you have a product and you list it on digital marketplace like paydotcom clickbank then other online marketr will sale your product as affiliates.
Generally it seems the same, but I think digital marketing is a little broader. I have seen LCD billboard, tv commercials?, cell phone ringers, text messages, and many other forms of "digital marketing."
i would class digital, to include anthing that is marketted via the media (TV, Radio, Mobile Phones Etc)
Radio and TV aren't digital really in a typical sense. The example someone else gave of a digital billboard is a great one though. It's one of the newer and hot concepts in advertising, and being able to adapt the display to individuals as opposed to a static billboard. You could tie media marketing to digital marketing in other ways though. For example, digital marketing might not involve the message itself appearing in a digital medium, but might instead encourage digital interaction online between the viewer/listener and the company's website. Other examples would be ATM ads that show up on the machine's screen, LCD signs / banners on top of cabs, if your cell phone company markets to you through text messages, etc.