well, ptc which stands for pay to click is usually a programs where the user will get paid to click ads or links, and they will get paid for every ad they click. so, the site emphasize the money seeker. for example, isabelmarco have a ptc program.
while ppc which stands for pay per click usually a service provided by a site, which allow the user to advertise their sites/ads. the user/advertiser will have to pay for every click on their ads they submitted to the site. for example, google have a ppc program named adword.