What is CAPTCHA for security purpose?

Question

What does CAPTCHA do as far as security issue is concerned? Registration form of many sites have this field but how it works?

Answer 1

Completely Automated Public Turing Test To Tell Computers and Humans Apart

It prevents hackers from posting forms using automatic scripts, by requiring the user to input data read from images which are difficult to read automatically. The text can also be in the form of a sound, as per @BeRecursive's comments See this site.

It is used for logins as well as on other data entry forms. Here on Stack Overflow, if you edit answers or questions a number of times, you will be prompted before further edits are accepted.

There are two main forms. One has a single combination of characters that the user has to enter, the other, such as on SO has two.

The CAPCHA with two words usually consists of a word known to the Web Application and a second word that it is trying to decipher. See this site (thanks @Piskvor) The first word is used for validating the user and the answers to the second word are compared to other users' answers for that word and in this way the probable meaning of the text is determined. This is performed as a public service to organisations such as Libraries and Public Archives that are scanning large numbers of historical documents. The Optical Character Recognition (OCR) is not perfect and sometimes the meaning cannot be determined. So the word is made available in the CAPTCHA of a participating website and the meaning is determined. This process has no affect on the user of the website as it is only the first word that is used to determine whether they are a robot.