Here's an excerpt from the Advertise page concerning Click Credits for anyone who isn't 100% sure about them like I was.
QUOTE
PPC stands for Pay Per Click, and is one of the most popular advertising methods in the global internet market. The concept is simple, you pay for each unique click your ad receives. In order to create PPC campaigns, you simply purchase P2L Click Credits. Once you have acquired Click Credits on your account, you can create as many PPC based ad campaigns as you wish and assign clicks from your Click Credits. As a quick example, if you were to purchase 1000 Click Credits, you could then create 1 PPC ad campaign and assign all 1000 credits, or create 10 campaigns with 100 credits each, or even 100 campaigns with 10 credits each.
CPM stands for Cost Per 1000 Impressions. This means that you pay a set amount every time your and is displayed on the site 1000 times, regardless of how many clicks you receive. CPM campaigns run on the same credit based system as PPC campaigns, only you need to purchase CPM Credits rather than Click Credits. Once again, you must first purchase CPM Credits and then you are able to create as many CPM campaigns as you wish, and then assign CPM credits to them.