The fundamental difference is that the purpose of "drip marketing" is to build a relationship rather than to secure a transaction. It is about cultivation of a prospect through the sales cycle, with the ultimate goal being not only repeat sales but advocacy (i.e. turning your customers into your sales team).

You need to be careful with getting too aggressive with it: there is a fine line between dripping and spamming. The key is to segment your list, define the specific needs/interests of the segments and then provide them with targeting communications that are focused on cultivating a relationship (not pushing a promotion).

Good luck!