CTR means Click Through Rate. It is percentage of how many people clicked on your ads for every 100 impressions, eCPM is average coast of click per thousand impressions.
CTR means click through ratio, because the higher times the surfer clicks the Adsense ad, higher is the payout to you.
Click-through Rate or CTR is a way of measuring the success of an online advertising campaign. A CTR is obtained by dividing the number of users who clicked on an ad on a web page by the number of times the ad was delivered (impressions). For example, if your banner ad was delivered 100 times (impressions delivered) and one person clicked on it (clicks recorded), then the resulting CTR would be 1 percent.