PPV traffic simply means 'pay per view' traffic. This means that the affiliate will pay the advertising network every time your website or advertisement is displayed to a viewer.
PPV ads are typically served as a result of software being installed on a users computer. If you advertise on a PPV ad network you are essentially advertising within software e.g. possibly a web browser or toolbar.
Pay per view traffic networks give away free software and internet tools to users by the millions. These users agree very clearly to receive keyword targeted advertisements for the privilege of using these valuable software or computer tools. The users are also instructed very clearly how to "opt out" of the network and remove the software and advertisements.