PPS stands for Pay Per Sale.
A PPS offer typically pays more per individual sale/signup, but it often has a lower associated conversion rate than its PPL (pay-per-lead) counterpart. This is because PPS offers require the user to enter their credit card information to classify as a sale.
In most cases, assuming the user enters their credit card information, the affiliate will still earn the PPS commission even on trials.
For additional terminology and key performance indicators for measuring success, please see: