CAPTCHA

Captcha stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." In a nutshell, it's a way to prove you're really a person and not a computer trying to break into a website or send a bunch of spam.

Basically, a captcha is a type of challenge-response test. It will show you some distorted letters and numbers, and you have to type them into a box. A computer probably wouldn't be able to figure out the distorted letters, but a human can. That way, the website can be sure that a real person is trying to do something on their website, not a computer.