CAPTCHAs

A program that can ensure whether the response is generated by a human, not from a machine, is called CAPTCHA. Major categories are CAPTCHA with complex background, BotBlock and BotDetect CAPTCHA, google CAPTCHAs, Facebook CAPTCHAs and overlapped CAPTCHAs. 

[http://gizmodo.com/5897661/google-finally-puts-captchas-to-good-use]