Conversion terms: Impressions, CTR, CPA, CPM

Click = An action taken after viewing an online ad or email message, usually leading to a view of the advertisers web site, or some other desired landing page.

Cookie = A small file stored by a web site on a users computer to track their usage of the web site and customize the site experience based on user tendencies. Cookies allow sites like Amazon to recommend new books by your favorite author.

Unique User = A view of your web site by a unique person. Multiple visits in that time period, by the same person, count as a single unique user. You can get this number from your server logs.

CPA / Cost Per Action = Total cost of marketing / Number of sales

CPC / Cost Per Click = Total cost of marketing / Number of clicks or website visits

Impressions = In PPC it means ad views. In Email Marketing it is also referred to as 'message opens' and means the number of times a user opened the email message and was exposed to the offering.

CPM / Cost Per 1000 Impressions = Total cost of marketing / (Impressions/1000)

CTR / Click Thru Ratio = Number of clicks / Number of impressions * 100

ROI / Return on Investment = Total Sales / Total cost of marketing * 100