When we sign up for services online, or log in to secure pages we are usually asked to provide a username and password. In order to disincentivise people writing software to sign up to things in bulk you are sometimes shown a picture of some letters which have been distorted in some way. This method of stopping fraudulent sign ups and spam is called a CAPTCHA.

CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart and is devised as a way of trying to ensure that the person signing up for a service or sending an enquiry via an online for is indeed a person and not an automated programme used by hackers.

Alan Turing (the T in CAPTCHA) was a pioneering British computer scientist why as well as working at Bletchley Park helping to crack the Enigma code, also devised a series of tests to determine whether tasks had been performed by a human or a computer.

In the ReCaptcha system the user types two distorted words. The words come from scans of books that are being digitised as part of the Google Books project. In this video Luis von Ahn explains the system.