Impressions: Refers to the number of times your ad was viewed by a user. Someone searches a keyword, your ad pops up and they, presumably, see it – your ad has made an impression. These are free in Pay-Per-Click campaigns, because they are counted separate from the number of times your ad is “clicked” by a user.
Click Through Rate (CTR): The rate at which people see your ad and decide to click it, which can be a measurement of the effectiveness of the ad. The percentage is calculated by clicks over impressions. Average CTR varies between industries and the product or service being advertised.
Conversion: A site visitor that becomes a paying customer.
Cost Per Click (CPC): What it costs each time a user clicks your ad. You want a good balance between the CPC and actual conversions. While a low CPC is good, one that is too low means you are not getting high quality leads; spending a little more for better conversions guarantees better ROI (Return on Investment).
Cost Per Lead: Once a user clicks on a PPC ad, they may have a look about your website and leave without any interaction. If they do interact with the site with intent to make a purchase, perhaps by visiting a Contact Us page or filling out a form, they are considered a “lead.” The Cost per Lead only counts the clicks that turn into leads, divided from your budget.