You regularly came across with this type of word verification technique in day to day of your life. Well folks its called CAPTCHA.
A CAPTCHA is a challenge-response program that protects websites against bots by generating and grading tests that humans can pass but computer programs cannot. The process involves a computer server asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Thus, it is also described as a reverse Turing test, because it is administered by a machine and targeted to a human. A common type of CAPTCHA requires that the user type letters or digits from a distorted image that appears on the screen.
The term "CAPTCHA" (based upon the word capture) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all from
). It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart." Carnegie Mellon University