What does a CAPTCHA do?
Humans can read the distorted text in CAPTCHA challenges* but current computer programs cannot.
A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot.
What does CAPTCHA mean?
CAPTCHA is an acronym for Completely Automated Public Turing Test To Tell Computers and Humans Apart. It was coined in 2000 by Carnegie Mellon University computer science research staff who invented CAPTCHA originally.
What is the difference between CAPTCHA and reCAPTCHA?
This is how the reCAPTCHA Project explains the difference:
ReCAPTCHA helps prevent automated abuse of your site (such as comment spam or bogus registrations) by using a CAPTCHA to ensure that only humans perform certain actions.
Generally a CAPTCHA is a single word, whereas a ReCAPTCHA is two words. The reCAPTCHA project page explains this in greater detail. There are research papers, in *.pdf format available for download on the Google ReCAPTCHA website.
Google purchased CAPTCHA in 2009 and describes usage and further background on reCAPTCHA FAQs:
ReCAPTCHA is a free CAPTCHA service that helps to digitize books, newspapers and old-time radio shows.
ReCAPTCHA is free
While free to use, including the API, be aware that ReCAPTCHA is not open source software.
ReCAPTCHA is best known for historic text digitization and spam filtering, which is an information security measure.
Answers to reCAPTCHA challenges are used to digitize textual documents… a combination of multiple OCR programs, probabilistic language models, and the answers from millions of humans on the internet, reCAPTCHA is able to achieve over 99.5% transcription accuracy at the word level….
OCR is an acronym. It means Optical Character Recognition. Compare the accuracy of standard OCR versus reCAPTCHA transcriptions of a medium quality scanned document on the reCAPTCHA digitization accuracy website. See some humorous reCAPTCHA examples from the official Google reCAPTCHA blog. Google announced an audio version of reCAPTCHA in 2009.
MailHide is another application, where potential for spam is reduced by requiring a reCAPTCHA challenge in order to disclose an otherwise partially obscured email address. More details are available in my post about MailHide from last month.
Recent research in the area of computer security led to some surprising discoveries about CAPTCHA and spam. Initially, it appeared that the CAPTCHA challenge had been defeated on a large scale, but localized very regionally. That was not true though. Human interaction of an unanticipated sort was still required to evade the CAPTCHA, on each and every spam comment and email that got through.
*Work continues on the original CAPTCHA project.