A captcha is a task that is designed to distinguish humans from machines in order to combat spam. It can take the form of text that is difficult to read, an arithmetic exercise or an image recognition task. Captchas should be easy for humans to understand and practically impossible for bots to solve.
The word ‘Captcha’ is an acronym for ‘Completely Automated Public Turing test to tell Computers and Humans Apart’.
This is a type of security measure that is used to prevent bots from performing actions on websites. Classic captchas require users to enter an often randomly generated combination of characters or letters into a field. Captchas can also take the form of solving simple maths problems: numbers are displayed in a handwritten format that are difficult for machines to read.
Advances in machine learning processes have led to machines gaining enhanced handwriting recognition capabilities, however, which has led to some captchas becoming obsolete.
Obviously a captcha will prevent a robot moving past it so SEOs must consider the content they put behind it. In addition it provides a barrier to the user and may affect conversions, form completion rate, usability and aesthetics.
There are a number of alternative approaches to distinguishing bots from real users. Google, for example, offers two different captcha variations for free use, and there are also several other providers besides Google.
On a somewhat less positive note, there seems to be no end in sight to the battle raging between developers of captchas and spambots.