Social networking sites employ a program called Captcha — as do many other sites to stop fraudsters from creating thousands of email IDs and then sending spam.
Captcha, which distorts the letters of a word in a way that can be identified only by a human, is widely employed by many companies to distinguish between humans and automated programs.
The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) 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.
To know more details, visit http://www.captcha.net - the official Captcha site or http://en.wikipedia.org/wiki/Captcha.