PPL stands for Pay Per Lead, it is another name for Cost Per Lead (CPL). Generally Advertisers tend to use this term more than Affiliates, but you may hear Affiliates mention this term instead of Cost Per Lead (CPL).
On WOW TRK we try to always use the same terminology, so we will always refer to campaigns as CPL and not PPL, however they are the same.
You can find a list of networks and programs that have CPL offers here.