CAPTCHA
What is a captcha?
A CAPTCHA test is designed to determine if an online user is really a human and not a bot. CAPTCHA is an acronym that stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." Users often encounter CAPTCHA and reCAPTCHA tests on the Internet. Such tests are one way of managing bot activity, although the approach has its drawbacks.
Although CAPTCHAs are designed to block automated bots, CAPTCHAs are themselves automated. They're programmed to pop up in certain places on a website, and they automatically pass or fail users.
How does CAPTCHA work?
Classic CAPTCHAs, which are still in use on some web properties today, involve asking users to identify letters. The letters are distorted so that bots are not likely to be able to identify them. To pass the test, users have to interpret the distorted text, type the correct letters into a form field, and submit the form. If the letters don't match, users are prompted to try again. Such tests are common in login forms, account signup forms, online polls, and e-commerce checkout pages.
The idea is that a computer program such as a bot will be unable to interpret the distorted letters, while a human being, who is used to seeing and interpreting letters in all kinds of contexts – different fonts, different handwritings, etc. – will usually be able to identify them. The best that many bots will be able to do is input some random letters, making it statistically unlikely that they will pass the test. Thus, bots fail the test and are blocked from interacting with the website or application, while humans are able to continue using it like normal. Advanced bots are able to use machine learning to identify these distorted letters, so these kinds of CAPTCHA tests are being replaced with more complex tests. Google reCAPTCHA has developed a number of other tests to sort out human users from bots.
What triggers a CAPTCHA test?
Some web properties just automatically have CAPTCHAs in place as a proactive defense against bots. Other times, a test may be triggered if user behavior seems to resemble a bot's behavior: if users request webpages or click hyperlinks at a far higher rate than average, for instance.