Completely Automated Public Turing Test to Tell Computers and Humans Apart
- trademarked by Carnegie Mellon University
When a website requires you to type in a word that appears in a graphic to prove that you're human and thus prevent bad web guys from automating the process, that's CAPTCHA.
Sometimes, a blogger will turn this option on for comments. So if you want to leave a comment, you first have to type in a word that's shown in an image file, w/c a bot has a hard time reading. (See, we humans are smart!)
Without CAPTCHA, a bad web guy can get a computer to automatically paste spam into a comments box or to sign up for a large number of free email accounts or register for sweepstakes without end.