(You're in "internet marketing" and you can't figure this out??? Or google it? Seriously?? )
PPC - is a payment structure, but often used to refer to contextual advertising like Adsense.You run ads (usually for various merchants) on your site, you're paid when someone clicks on an ad
Affiliate marketing - is a form of marketing. you join specific programs and post ads and/or links. CAN be PPC also, but you're usually paid per sale or per action (i.e. submitting a form.)