We have all seen them on website logins and form submissions, Captcha is a simple but effective layer of security designed to protect websites by differentiating between humans (you & me) and Bots (Automated processes that trawl the web for various purposes).
Some of those purposes are benign, (Google use them to build the index that forms the basis of their searches) others are used to harvest email addresses for Spam campaigns; potential hackers use them to look for web sites with known security flaws or insecure log-ins.
So Captcha provides a useful addition to the security we provide for your website, by providing protection from Bots that in some forms can endanger the security of your site.
Captcha works by generating and partially obscuring simple alpha numeric recognition tests, that humans are easily capable of solving; while exploiting the limitation of Bot technologies such as OCR (Optical Character Recognition) that should make the test almost insurmountable for Bots; thus providing security by filtering out the Bots.
Captcha is an acronym, standing for Completely Automated Public Turing Test.
Captcha is often referred to as a reverse Turing test, as it is used to identify humans, the standard Turing test was originally devised as a method to help researchers assess the success or otherwise of artificial intelligence in replicating human intelligence. (Named after Alan Turing the famous scientist who was the inspiration for the 2014 film, The Imitation Game).