CAPTCHA or CAPTCHA Challenge is a type of Web Challenge. CAPTCHA stands for ‘Completely Automated Public Turing test to tell Computers and Humans Apart’. It is challenge intended to differentiate between computers and humans. Computers generally are unable to solve the CAPTCHA and state the word and letters, while humans are.
CAPTCHA is used to mitigate DDoS attacks, as legitimate users are able to pass it, while attacking computers cannot. Nevertheless, CAPTCHA is not the most popular DDoS web challenge because it is very intrusive and has a negative effect itself.
Related entries: Cookie Validation, Web Challenges, Web Challenge Spectrum.