Both the terms are being used synonymously these days but they certainly difer from each other.
PPC sites are those such as Adsense, Adbrite etc which pays its publishers when someone clicks on the ad displayed on their website.
PTC refers to sites of kind bux.to which most of them are spam. There are tons of such sites but most of them are scam. Few legal ones that I know are bux.to and clixsense.com