The different between CPA, CPL, CPS and CPC
. CPA ( Cost Per Action) : model pays you for specific actions and it can be simple form submissions, downloads, surveys,etc.
. CPL (Cost Per Lead) : Model pays for leads and it's usually a "signup" that involves email or credit card verification (because it makes the lead more valuable).

. CPS (Cost Per Sale) : Model pays for actual sales and it meas they share a percentage of sale value with you.
. CPC (Cost Per Click) : Models as you probably know pays for "Clicks" - no matter whether you referral traffic helped them generate a lead/sale or not.