For example, we now support multiple pricing models. Different web publishers charge different rates for their ads. Some charge on a CPM (impressions), some on the number of days, and some on CPC (clicks) basis. Sometimes ads are exclusive: no other advertisement can be sold for days already purchased. We have also added support for a CPM bidding system that is similar to Google AdWords and gives a higher priority to ad purchases that come in at a higher CPM rate.