***CTR is click through rate::: For standard content ads, the number of times a user clicked on an ad. For link units, the number of clicks received by ads on a page resulting from a link click.

***CPC is::: The cost-per-click (CPC) is the amount you earn each time a user clicks on your ad. In your reports, CPC is calculated by dividing the estimated earnings by the number of clicks received.

in other words , CTR is how good the ads is, more high the CTR more click they got , CPC is the cost you must spend for the ads per click