CAPTCHA, which stands for Completely Automated Public Turing test to tell Computers and Humans Apart, is a technique adopted by Yahoo, Google, and Microsoft (News - Alert) among other service providers to prevent automated software programs from posing as humans and signing up for new accounts.It presents text that is easy for humans to comprehend but difficult for automated programs and as a result protects Web sites from bots. The first CAPTCHA was invented at the Carnegie Mellon University to be used by Yahoo. CAPTCHAs have other applications like preserving the authenticity in online polls, preventing comment spam in blogs and preventing dictionary attacks in password systems among others.