Click-through rate (CTR) is a way of measuring the success of an online advertising campaign for a particular website as well as the effectiveness of an email campaign by the number of users that clicked on a specific link.
The purpose of click-through rates is to capture customers' initial response to websites. Most commercial websites are designed to elicit some sort of action, whether it be to buy a book, read a news article, watch a music video, or search for a flight. People generally don't visit a website with the intention of viewing advertisements, just as people rarely watch TV with the purpose of consuming commercials. 
Marketers want to know the reaction of the web visitor. Under current technology, it is nearly impossible to fully quantify the emotional reaction to the site and the effect of that site on the firm's brand. One piece of information that is easy to acquire, however, is the click-through rate. The click-through rate measures the proportion of visitors who initiated action with respect to an advertisement that redirected them to another page where they might purchase an item or learn more about a product or service. Here we have used "click their mouse" on the advertisement (or link) because this is the generally used term, although other interactions are possible.
The click-through rate is the number of times a click is made on the advertisement divided by the total impressions (the times an advertisement was served):
Click-through rate (%) = Click-throughs (#) / Impressions (#)
Online Advertising CTR 
The click-through rate of an advertisement is defined as the number of clicks on an ad divided by the number of times the ad is shown (impressions), expressed as a percentage. For example, if a banner ad is delivered 100 times (100 impressions) and receives one click, then the click-through rate for the advertisement would be 1%.
Click-through rates for banner ads have fallen over time. When banner ads first started to appear, it was not uncommon to have rates above five percent. They have fallen since then, currently averaging closer to 0.2 or 0.3 percent. In most cases, a 2% click-through rate would be considered very successful, though the exact number is hotly debated and would vary depending on the situation. The average click-through rate of 3% in the 1990s declined to 0.1%-0.3% by 2011. Since advertisers typically pay more for a high click-through rate, getting many click-throughs with few purchases is undesirable to advertisers. Similarly, by selecting an appropriate advertising site with high affinity (e.g., a movie magazine for a movie advertisement), the same banner can achieve a substantially higher CTR. Though personalized ads, unusual formats, and more obtrusive ads typically result in higher click-through rates than standard banner ads, overly intrusive ads are often avoided by viewers.
Email CTR 
An email click-through rate is defined as the number of recipients who clicked one or more links in an email and landed on the sender's website, blog, or other desired destination. More simply, email click-through rates represent the number of clicks that your email generated.
Most email marketers use this metrics along with open rate, bounce rate and other metrics, to understand the effectiveness and success of their email campaign.  In general there is no ideal click-through rate. This metric can vary based on the type of email sent, how frequently emails are sent, how the list of recipients is segmented, how relevant the content of the email is to the audience, and many other factors.  Even time of day can affect click-through rate. Sunday appears to generate considerably higher click-through rates on average when compared to the rest of the week.
See also 
- Abandonment rate
- Ad serving
- Banner blindness
- Click fraud
- Cost per action
- Cost per click
- Cost per thousand
- Impression (online media)
- View-through rate
- Open rate
- Email marketing
- American Marketing Association Dictionary. http://www.marketingpower.com/_layouts/Dictionary.aspx. Retrieved 2012-11-02. The Marketing Accountability Standards Board (MASB) endorses this definition as part of its ongoing Common Language: Marketing Activities and Metrics Project.
- Farris, Paul W.; Neil T. Bendle; Phillip E. Pfeifer; David J. Reibstein (2010). Marketing Metrics: The Definitive Guide to Measuring Marketing Performance. Upper Saddle River, New Jersey: Pearson Education, Inc. ISBN 0-13-705829-2. The Marketing Accountability Standards Board (MASB) endorses the definitions, purposes, and constructs of classes of measures that appear in Marketing Metrics as part of its ongoing Common Language: Marketing Activities and Metrics Project.
- Google AdWords Help: Clickthrough rate (CTR)
- Yahoo Search Marketing Glossary
- IAB Click Measurement Guidelines
- Google Analytics Help: What's the difference between clicks, visits, visitors, pageviews, and unique pageviews?
- IAB Glossary of Interactive Advertising Terms
- Stern, Andrew (February 1, 2010). "8 ways to improve your click-through rate". iMedia Connection. Retrieved February 7, 2010.
- Li, Hairong; Leckenby, John D. (2004). "Internet Advertising Formats and Effectiveness". Center for Interactive Advertising. Retrieved 26 February 2010.
- "How to Price and Place Your Ads". Retrieved 21 October 2011.
- US application 20,090,157,495
- "Email Campaign Performance Metrics Definitions". Retrieved December 18, 2012.
- Kevin Gao. "Click Through Rates: Click Through Rates Numbers and Their Meaning". Retrieved December 18, 2012.
- "The Basics of Email Metrics: Are Your Campaigns Working?". October, 2008.
- John Arnold (April, 2011). "Calculating the Click-through Rate for Your E-Mail Marketing Campaign from E-Mail Marketing for Dummies, 2nd Ed.".
- "Email marketing metrics: Click through rate (CTR) relevant to email marketing measurement". January 17, 2010.
- "Average Email Click-Through Rate". Retrieved December 20, 2012.
- Pete Prestipino (July 21, 2011). "EMail Marketing Metrics 2011".
- Matt McGee (July 23, 2012). "E-mail Open Rates Declining, Click-Through Rates Rising [Study]".
- David Moth. "Email marketing stats: consumers open just 20% of messages". Text " date July 24, 2012" ignored (help)
Further reading 
- Sherman, Lee and John Deighton, (2001), "Banner advertising: Measuring effectiveness and optimizing placement," Journal of Interactive Marketing, Spring, Vol. 15, Iss. 2.
- Ward A. Hanson and Kirthi Kalyanam, (2007), Internet Marketing and eCommerce, Chapter8, Traffic Building, Thomson College Pub, Mason, Ohio.