An ad that loads in a small, separate browser window while you are waiting for a Web page to load. There are two types of interstitial ads, namely the pop-up and the inline.
 
The pop-up is the most widely used form of interstitial advertising. It is activated when a user either arrives at or leaves a triggering web page. A small window “pops” that advertises a specific product or service, while the intended web page loads in the background.
 
Inline interstitials are more advanced and are used in rich-media environments making use of Java, Shockwave or VRML (Virtual Reality Markup Language).
 
Some studies show that users are more likely to click through interstitial ads than banner ads. The down side is that some users complain they slow down the load time for destination pages. Like all things in Internet marketing, you should test the results for yourself.