Interstitial ads and Inline ads are the types of ads offered by Adbrite (PPC Programme). Interstitial ads are like temporary frames appearing over the top of the web page asking visitors to view the advertised site. The publisher receives a payment every time a visitor does this. Inline ads are underlined words in the text content. When a mouse pointer is scrolled over an inline ad a small pop-up box appears with an ad inside - the publisher receives payment every time one of these small pop-ups is clicked. Network ads are displayed ads that looks very similar to the looks of Adsense ads.The publisher's receives payments everytime a visitor clicks an ad. The networks are not contextual and relaible enough because some days these ads dont show up at all. Thanks and Hope this helps!