In a simple language click through rate is nothing but the ratio of users who are clicking on a specific link to the total numbers of users who view the page,Email or advertisement.

Basically click through rate is used to measure the success rate of a campaign for a particular website.

Click through rate of a campaign can vary tremendously.The main purpose of the click through rate is to measure the ratio of clicks to impressions(views) of an online ad or a campaign.

The higher the ctr of a particular website the more effective will be the campaign of that particular website.

The formula for the click through rate is number of click throughs per number of impressions the whole multiplied by 100.
This will give you the ctr percentage of that particular website.