So what's the deal with "Captcha"?

In a CAPTCHA test (an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart," also sometimes spelled in lowercase), an image of letters is dynamically generated. The letters, because they're part of an image and not text (e.g. text that you could cut and paste), are difficult for a spambot or other computer program to read. Yet, a person has little trouble reading the letters in a captcha image.

Using a captcha test on a website is a great way to ensure, for instance, that a person and not a spambot is filling out a web form. Also, a captcha can make it difficult for a person to continuously resubmit form information and overwhelm the form's purpose.

Want to learn how to implement it?