Drip Marketing refers to the effort of creating junks of content that you "drip" into a pool of users in order to get them from A to Z, un-informed to informed, contact to lead to customer etc. Drip marketing is ideally highly automated and is using a CRM data as data source.
Some implementations could be:
Promotional: create promote multiple promotional messages that lead users to convert.
Top-of-Mind: send repeat information such as white papers or marketing material to stay top-of-mind.
Re-Engagement: follow up with old customers to get them again interested about your products and services.
Training: create a training program, eventually for staff or partners.
Moving target audiences between different channels is another good tactic for example converting Facebook fans to Twitter followers or email subscribers etc.
Drip marketing campaigns should be designed for your respective audience or segment and measured by evaluating the performance and conversion from one drip to another.
Here is an example Drip Marketing campaign:
Send email to list of customers
The ones who opened the first email receive and e-book link in a couple of days.
The ones that downloaded the e-book get an offer for a product demo.
The ones who did not open the first email receive an email with a video explainer in a couple of days.
The ones that watched the video receive the e-book link email etc..
The ones that did not open any email fall of the list.