In digital marketing (as in all areas of marketing) the message is designed to drive sales or directly increase revenue in some way. For example, the marketing message of a large online book store might be that they can offer the largest selection and cheapest prices because of their bulk buying power.
In digital public relations (and Public Relations as a whole) the message isn’t designed to be directly promotional. PR messages can vary from showing that a company cares about a specific cause (or show core values of the company) to news messages put out by the company.