Pay per impression is not exactly the right terminology. the correct is CPM, Cost per thousand, CPM is measurable unit and $5CPM is saying that for 1000 shown ads you get paid $5. CPM is used with popups or just showing ads. Popups are more agressive so there are higher rates.

Than advertiser pay per impression of the Ad and you publisher show ads (banners or popups or popunders) on your site.

Go to www.pay-per-impression.com and there you get some good links.