PPC - Pay per click are when a potential customer clicks on the link that you are advertising for the merchant. Example Google adwords
PPC (2nd kind, not sure what is the common name) - You allow merchants to place links on your website and when a potential customer clicks on that link you get paid. (Example" Google Adsense)
PTC sites - You join a site for free and are paid to click on advertisements. (Ususally by other members of the site promoting their other PTC sites) (There are many sites like these but they only pay 1/4 cent per click which is a waste of time but adbux pays at least 1 cent, which means it saves me 3 clicks every time)
PTR sites - This can be joined in PTC sites or separate. You read emails and will paid according to site's payment scheme. Or you join a site which sends you paid emails for you to read. (Example:
http://www.emailcashpro.com or
http://www.e-mailpaysu.com) Both sites pays at least 2 cents per email.
