The term CAPTCHA stands for 'Completely Automated Public Turing Test to tell Computer and Humans Apart'.

It is a word verification test that is generated by a server to ensure that the response is generated only by human and not by a computer or bot.

CAPTCHA was first created by Nicholas J. Hopper, John Langford, Luis Von Ahn and Manuel Blum.

CAPTCHA alphabets and numbers are arranged in such a pattern that currently the computer software cannot read those alphabets and number but for humans it is quite easy to figure out the letter and numbers in CAPTCHA and pass that test.

The main purpose of CAPTCHA is to protect websites from automated attacks generated by computers called bots. CAPTCHA is usually used by websites like hotmail, yahoo and Gmail.

These websites use CAPTCHA service to ensure that the forms being filled are actually generated and filled by humans and not by a bot or automated software.

CAPTCHA is also used by websites offering online polls to ensure that all the votes being casted are from humans and an automated software is not trying to manipulate the voting system.

CAPTCHA can also serve to protect websites such as Hotmail, yahoo and Gmail against email spam and denial of service attacks.

Currently a correctly designed and deployed CAPTCHA system cannot be bypassed by a computer because a correctly designed CAPTCHA would not permit more than one attempts at one CAPTCHA.