Reach: It shows the total number of people to how your ad/promotion was shown at least once during that period.
Impressions: It indicates the number of times your ad/promotion was displayed.
- SEO increases your website traffic resulting in better ranking in the search results page.
CTR [Click Through Rate]: It is the ration of users who click on a specific link to the total number of users who have viewed your webpage, email, or advertisement.
- It is used to measure the success of your online advertising or your website.
Source: Wikipedia, the free encyclopedia.