A CAPTCHA or Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart."

Characteristics
The challenges for the creation of captcha are:
* Current software is unable to solve accurately.
* Most humans can solve
* Does not rely on the type of CAPTCHA being new to the attacker.

CAPTCHAs rely on difficult problems in artificial intelligence

Applications
CAPTCHAs are used to prevent automated software from performing actions which degrade the quality of service of a given system. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam. in stopping automated posting to blogs, forums and wikis, whether as a result of commercial promotion, or harassment and vandalism, they are very much handy. a CAPTCHA can enforce automated usage policies as set by the administrator when certain usage metrics exceed a given threshold.

Accessibility:
sites implementing CAPTCHAs may provide an audio version of the CAPTCHA in addition to the visual method for accessibility reasons..

For more details, visit this link:
http://en.wikipediawiki/Captcha