A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't. And that is the reason this software is being modified a little bit and is being used to recognise the words in the millions of books which was written before the advent of computer so that they can be preserved.
The term CAPTCHA was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. At the time, they developed the first CAPTCHA to be used by Yahoo.