CAPTCHA is an acronym for Completely Automated Public Turing test to tell Computers and Humans Apart.
It is a type of visual confirmation commonly used in website forms to ensure the submission of an entry is not automatically done by a computer / bot.
Basically, CAPTCHA is an image with a text shown on it. The user must type the same characters as the ones on the image to verify he or she is human.
It is often used in forums and contact us pages to prevent entries from spam bots. A very popular CAPTCHA application is the reCAPTCHA from Google.