Clickthrough rate (CTR) is the number of clicks your ad receives divided by the number of times your ad is shown (impressions). Your ad and keyword each have their own CTRs, unique to your own campaign performance.
A keyword's CTR is a strong indicator of its relevance to the user and the overall success of the keyword. For example, a well targeted keyword that shows a similarly targeted ad is more likely to have a higher CTR than a general keyword with non-specific ad text. The more your keywords and ads relate to each other and to your business, the more likely a user is to click on your ad after searching on your keyword phrase.
A low CTR may point to poor keyword performance, indicating a need for ad or keyword optimization. Therefore, you can use CTR to gauge which ads and keywords aren't performing as well for you and then optimize them.
CTR is also used to determine your keyword's Quality Score. Higher CTR and Quality Score can lead to lower costs and higher ad position.
2) eCPM = effective Cost Per Thousand Impressions
From a publisher's perspective, the effective cost-per-thousand impressions (eCPM) is a useful way to compare revenue across different channels and advertising programs. It is calculated by dividing total earnings by the number of impressions in thousands. For example, if a publisher earned $180 from 45,000 impressions, the eCPM would equal $180/45, or $4.00. However, please keep in mind that eCPM is a reporting feature that does not represent the actual amount paid to a publisher.
3) CPC = Cost Per Click
Pay per click (PPC) is an Internet advertising model used on search engines, advertising networks, and content sites, such as blogs, in which advertisers pay their host only when their ad is clicked. With search engines, advertisers typically bid on keyword phrases relevant to their target market. Content sites commonly charge a fixed price per click rather than use a bidding system.
Websites that utilize PPC ads will display an advertisement when a keyword query matches an advertiser's keyword list, or when a content site displays relevant content. Such advertisements are called sponsored links or sponsored ads, and appear adjacent to or above organic results on search engine results pages, or anywhere a web developer chooses on a content site.
Although many PPC providers exist, Google AdWords, Yahoo! Search Marketing, and Microsoft adCenter are the three largest network operators, and all three operate under a bid-based model. Cost per click (CPC) varies depending on the search engine and the level of competition for a particular keyword.
The PPC advertising model is open to abuse through click fraud, although Google and other search engines have implemented automated systems to guard against abusive clicks by competitors or corrupt web developers.