If you're an advertiser: CPC = you pay for every person that clicks on your ad and visits your site CPA = you pay for every person that clicks on your ad and buys from you / signs up for your newsletter / whatever your goal is So CPAs are usually higher cost, but are more easy to manage from an advertiser ROI point of view, and for high quality publisher sites can often generate more revenue. CPCs are simpler to measure and simpler to implement and put the onus on the advertiser to optimise.
Cost per click is an advertising model where in the advertiser pays for each click or impressions or unique IP that enters the website. In comparison, Cost per Action is where the advertiser pays if somebody orders, purchase or joins the site. For more info, you may ask more. Hope this helps.
In the Cost Per Click advertising model, you will be charged for every click on your ad. You will not have to pay anything if users see your ad but do not click on it. Cost per click is determined by the following formula: Cost/Clicks It is a type of online advertising model in which return or payment is based on how many times the ad is clicked. Per-click prices can range from 0.10 cents to $10 depending on the popularity of the content or the advertising agreement. Cost Per Action or CPA (sometimes known as Pay Per Action or PPA) is an online advertising pricing model, where the advertiser pays for each specified action (a purchase, a form submission, and so on) linked to the advertisement. When referring to banner ads, CPA is the cost the advertiser pays to the site publisher each time a specific, visitor-initiated action occurs, eg, the completion of a sale.